An In Silico Study of the Antioxidant Ability for Two Caffeine Analogs Using Molecular Docking and Quantum Chemical Methods

molecules Article An In Silico Study of the Antioxidant Ability for Two Caffeine Analogs Using Molecular Docking and Quantum Chemical Methods Josivan da Silva Costa 1,2,3 , Ryan da Silva Ramos 2,3 , Karina da Silva Lopes Costa 2, Davi do Socorro Barros Brasil 4, Carlos Henrique Tomich de Paula da Silva 5, Elenilze Figueiredo Batista Ferreira 2, Rosivaldo dos Santos Borges 1, Joaquín María Campos 6, Williams Jorge da Cruz Macêdo 1,3 and Cleydson Breno Rodrigues dos Santos 2,3,6,* 1 Postgraduate Program in Biotechnology and Biodiversity-Network BIONORTE, Federal University of Pará, Rua Augusto Corrêa, 01, Belém, Pará66075110, Brazil; [email protected] (J.d.S.C.); [email protected] (R.d.S.B.); [email protected] (W.J.d.C.M.) 2Laboratory of Modeling and Computational Chemistry, Department of Biological Sciences, Federal University of Amapá, Rod. Juscelino Kubitschek, Km 02, s/n, Macapá, Amapá68902-280, Brazil; [email protected] (R.d.S.R.); [email protected] (K.d.S.L.C.); [email protected] (E.F.B.F.) 3Laboratory of Molecular Modeling and Simulation System, Federal Rural University of Amazônia, Rua João Pessoa, 121, Capanema, Pará68700-030, Brazil 4Institute of Technology, Federal University of Pará, Av. Augusto Corrêa, 01, Belém, Pará66075-900, Brazil; [email protected] 5Computational Laboratory of Pharmaceutical Chemistry, Faculty of Pharmaceutical Sciences of Ribeirão Preto, São Paulo 14040-903, Brazil; [email protected] 6Department of Pharmaceutical and Organic Chemistry, University of Granada, Campus of Cartuja, 18071 Granada, Spain; [email protected] *Correspondence: br[email protected]; Tel.: +55-96-4009-2699 Received: 12 August 2018; Accepted: 11 October 2018; Published: 29 October 2018   Abstract: The antioxidant activity of molecules constitutes an important factor for the regulation of redox homeostasis and reduction of the oxidative stress. Cells affected by oxidative stress can undergo genetic alteration, causing structural changes and promoting the onset of chronic diseases, such as cancer. We have performed an in silico study to evaluate the antioxidant potential of two molecules of the zinc database: ZINC08706191 (Z91) and ZINC08992920 (Z20). Molecular docking, quantum chemical calculations (HF/6-31G**) and Pearson’s correlation have been performed. Molecular docking results of Z91 and Z20 showed both the lower binding affinity (BA) and inhibition constant (Ki) values for the receptor-ligand interactions in the three tested enzymes (cytochrome P450—CP450, myeloperoxidase—MP and NADPH oxidase—NO) than the control molecules (5-fluorouracil—FLU, melatonin—MEL and dextromethorphan—DEX, for each receptor respectively). Molecular descriptors were correlated with Ki and strong correlations were observed for the CP450, MP and NO receptors. These and other results attest the significant antioxidant ability of Z91 and Z20, that may be indicated for further analyses in relation to the control of oxidative stress and as possible antioxidant agents to be used in the pharmaceutical industry. Keywords: antioxidant potential; molecular descriptors; molecular docking; binding free energy; free radicals; oxidative stress Molecules 2018,23, 2801; doi:10.3390/molecules23112801 www.mdpi.com/journal/molecules Molecules 2018,23, 2801 2 of 17 1. Introduction Oxidants play a key role in maintaining the redox homeostasis of cells. However, in large quantities an imbalance can be triggered. Reactive Oxygen Species (ROS) resulting from aerobic respiration are examples of extremely unstable oxidants that can collide with other species (molecules or biomacromolecules), causing their transformation (oxidative damages) and the increased oxidative damage causes cell stress, known as oxidative stress [1,2]. Several chronic diseases such as diabetes, neurodegenerative and cardiovascular diseases, and cancer can be caused by increased oxidative stress [3]. This occurs from the activation of various transcription factors, which can express hundreds of different genes, such as growth factor promoters and inflammatory cytokines, which lead to the activation of inflammatory pathways transforming a normal cell into a cancerous one [4]. Thus, maintenance of redox homeostasis and reduction of oxidative stress depend on the efficiency of antioxidant present in the cell, since the first and second defense barriers (antioxidant enzymes and proteolytic and lipolytic enzymes, respectively) have already been overcome [ 5 ]. Some enzymes, such as cytochrome P450 (CP450), lipoxygenase (LO), myeloperoxidase (MP), NADPH oxidase (NO) and xanthine oxidase (XO) that are known to generate ROS during the metabolism of arachidonic acid and their inhibitions break the ROS production cycle with the consequent reduction of the oxidative stress and maintenance of redox homeostasis [ 6 ]. The increase of the oxidative stress mediated by ROS may lead to the appearance of several diseases, including cancer. Therefore, the search of agents that maintains the balance of redox homeostasis (antioxidants) has an important role in the discovery of molecules that prevent and halt the growth of cancer cells via reduction of the oxidative stress [3–6]. The literature shows that molecular docking has been an important tool for studies of receptor-ligand interaction in the inhibition of enzymes related to antioxidant activity. This technique has clarified doubts and pointed out clarifications about the possible region of the receptor where the activity occurs, what amino acid residues are involved in the interactions and what atoms are directly interacting with the ligand [7]. Molecular docking has aided in the elucidation of the antioxidant mechanism of compounds submitted to biological tests such as new chromeno-carbamodithioates derivatives. These compounds have been evaluated for their antioxidant activity in the cyclooxygenase-2 enzyme, using the GOLD program to assess the full range of ligand flexibility and the rotational flexibility of selected receptor hydrogens, and the Autodock 4.0 program to check out the binding free energy and inhibition constant (Ki) concerning the interaction of ligands with the receptor [ 8 ]. Molecular docking study at the PPAR α / γ receptor has been used to evaluate the agonist and the antioxidant activity of a novel structural class of coumarin-chalcone fibrates using AutoDock 4.2.6 program [9]. As well as molecular docking analysis, significant importance can be attributed to obtaining molecular descriptors and information obtained from chemical-quantum calculations. These data assist in the elucidation of various physical and chemical properties resulting from the different classes of existing compounds [ 10 ]. A study on the antioxidant activity of 4-hydroxyphenyl substituted thiopyrimidines has been performed using the Gaussian 9 program to obtain the energies of molecular orbitals (HOMO e LUMO) and evaluation of the electron donor and acceptor character of the chemical species under analysis [11]. In similar studies, the antioxidant activity was evaluated for 1,3,4-thiadiazole derivatives [ 12 ] and for major chemical constituents present in the leaves of the Curatella americana Linn [ 13 ]. In both studies, molecular descriptors such as the dipole moment, polarizability, chemical hardness/softness, electronegativity and molecular orbital energies have been calculated, analyzed and the results used to evaluate the reactivity and stability of the species studied in relation to antioxidant activity. In this manuscript, two caffeine analogs of the zinc database ZINC08706191 (Z91) and ZINC08992920 (Z20), proposed by Costa et al. (2018) [ 14 ], have been tested in silico to evaluate the antioxidant potential via molecular docking with five enzymes (CP450, LO, MP, NO and XO). They have been compared with known molecules that were used as positive controls, such as: 5-fluorouracil Molecules 2018,23, 2801 3 of 17 (FLU), zileuton (ZIL), melatonin (MEL), dextromethorphan (DEX) and febuxostat (FEB), for each receptor, respectively. Quantum chemical calculations (HF/6-31G**) and Pearson’s correlation were performed for compounds studied here. 2. Results and Discussion 2.1. Evaluation of Molecular Docking Data about validation protocols for molecular docking can be seen in Figure 1. According to literature, the RMSD values expressing the relationship between the calculated X crystallographic data of the complexed ligand must be less than 2.0 Å [ 14 – 16 ]. The similarity in the overlapping of crystallographic poses (orientation + conformation, cyan) and calculated (yellow) was obtained via molecular docking and graphically displays a low RMSD value, what characterizes good results according to literature (see Figure 1). These results attest that the protocols used can be applied in the molecular docking analyzes between the receptors of the antioxidant activity and ligands. The binding free energies ( ∆ G) for the molecules evaluated at each receptor are shown in Figure 2, and these values were used to classify the best poses obtained in the molecular docking analyses. Only the smallest ∆ G values for the best poses are shown. The larger the peaks, the lower the ∆ G and consequently the more significant the interaction between the receptor and the ligands for the antioxidant ability. Molecules 2018, 23, x 3 of 17 5-fluorouracil (FLU), zileuton (ZIL), melatonin (MEL), dextromethorphan (DEX) and febuxostat (FEB), for each receptor, respectively. Quantum chemical calculations (HF/6-31G**) and Pearson’s correlation were performed for compounds studied here. 2. Results and Discussion 2.1. Evaluation of Molecular Docking Data about validation protocols for molecular docking can be seen in Figure 1. According to literature, the RMSD values expressing the relationship between the calculated X crystallographic data of the complexed ligand must be less than 2.0 Å [14–16]. The similarity in the overlapping of crystallographic poses (orientation + conformation, cyan) and calculated (yellow) was obtained via molecular docking and graphically displays a low RMSD value, what characterizes good results according to literature (see Figure 1). These results attest that the protocols used can be applied in the molecular docking analyzes between the receptors of the antioxidant activity and ligands. The binding free energies (ΔG) for the molecules evaluated at each receptor are shown in Figure 2, and these values were used to classify the best poses obtained in the molecular docking analyses. Only the smallest ΔG values for the best poses are shown. The larger the peaks, the lower the ΔG and consequently the more significant the interaction between the receptor and the ligands for the antioxidant ability. Figure 1. Data obtained in the validation of the molecular docking protocols for the receptors cytochrome P450 (CP450), lypoxygenase (LO), myeloperoxidase (MP), NADPH oxidase (NO) and xanthine oxidase (XO). Figure 1. Data obtained in the validation of the molecular docking protocols for the receptors cytochrome P450 (CP450), lypoxygenase (LO), myeloperoxidase (MP), NADPH oxidase (NO) and xanthine oxidase (XO). Molecules 2018,23, 2801 4 of 17 Molecules 2018, 23, x 4 of 17 Figure 2. Binding free energy values resulting from the molecular docking between the molecules and receptors evaluated. Control molecules for CP450, LO, MP, NO and XO were 5-fluorouracil (FLU), zileuton (ZIL), melatonin (MEL), dextromethorphan (DEX) and febuxostat (FEB), respectively. Figure 3 shows binding affinity data values for the selected molecules from the ΔG values. The tested molecules (Z91 and Z20) showed very similar binding affinity levels at the CP450, MP and NO receptors. The binding affinity values at these three receptors for the molecules tested are higher than control 1 and lower than control 2. The molecules tested had values of non-significant binding affinities (positive—not shown) in the LO and XO receptors, and these were excluded to subsequent analyzes. Figure 3. Binding affinity provided by AutoDock/Vina software of the tested molecules (Z20 and Z91). Control 1/Control 2 molecules for CP450, LO, MP, NO and XO were FLU/S-warfarin, ZIL/protocatechuic acid, MEL/N-acetyl-D-glucosamine, DEX/ adenosine-5’-diphosphate and FEB/hypoxanthine, respectively. Figure 4 shows the interactions data of the tested (Z20 and Z91) and control (FLU) molecules with the CP450 receptor. Five amino acid residues (PRO367, ALA 103, LEU366, PHE 114 and ILE99) were common to Z20, Z91 and FLU (indicated by the blue in diagram A). This shows the degree of correspondence between the control and the molecules tested inferring that these may have potential antioxidant ability. The Z20 and Z91 molecules with lower number of interactions (seven and six, respectively) showed a BA (−7.8 kcal mol−1 and −7.5 kcal mol−1, respectively) higher than control 1 (FLU) with BA = −9.8 kcal.mol−1 (eight interactions). It is possible to verify the tendency of the BA value to decrease on increasing the number of interactions. Figure 2. Binding free energy values resulting from the molecular docking between the molecules and receptors evaluated. Control molecules for CP450, LO, MP, NO and XO were 5-fluorouracil (FLU), zileuton (ZIL), melatonin (MEL), dextromethorphan (DEX) and febuxostat (FEB), respectively. Figure 3shows binding affinity data values for the selected molecules from the ∆ G values. The tested molecules (Z91 and Z20) showed very similar binding affinity levels at the CP450, MP and NO receptors. The binding affinity values at these three receptors for the molecules tested are higher than control 1 and lower than control 2. The molecules tested had values of non-significant binding affinities (positive—not shown) in the LO and XO receptors, and these were excluded to subsequent analyzes. Molecules 2018, 23, x 4 of 17 Figure 2. Binding free energy values resulting from the molecular docking between the molecules and receptors evaluated. Control molecules for CP450, LO, MP, NO and XO were 5-fluorouracil (FLU), zileuton (ZIL), melatonin (MEL), dextromethorphan (DEX) and febuxostat (FEB), respectively. Figure 3 shows binding affinity data values for the selected molecules from the ΔG values. The tested molecules (Z91 and Z20) showed very similar binding affinity levels at the CP450, MP and NO receptors. The binding affinity values at these three receptors for the molecules tested are higher than control 1 and lower than control 2. The molecules tested had values of non-significant binding affinities (positive—not shown) in the LO and XO receptors, and these were excluded to subsequent analyzes. Figure 3. Binding affinity provided by AutoDock/Vina software of the tested molecules (Z20 and Z91). Control 1/Control 2 molecules for CP450, LO, MP, NO and XO were FLU/S-warfarin, ZIL/protocatechuic acid, MEL/N-acetyl-D-glucosamine, DEX/ adenosine-5’-diphosphate and FEB/hypoxanthine, respectively. Figure 4 shows the interactions data of the tested (Z20 and Z91) and control (FLU) molecules with the CP450 receptor. Five amino acid residues (PRO367, ALA 103, LEU366, PHE 114 and ILE99) were common to Z20, Z91 and FLU (indicated by the blue in diagram A). This shows the degree of correspondence between the control and the molecules tested inferring that these may have potential antioxidant ability. The Z20 and Z91 molecules with lower number of interactions (seven and six, respectively) showed a BA (−7.8 kcal mol−1 and −7.5 kcal mol−1, respectively) higher than control 1 (FLU) with BA = −9.8 kcal.mol−1 (eight interactions). It is possible to verify the tendency of the BA value to decrease on increasing the number of interactions. Figure 3. Binding affinity provided by AutoDock/Vina software of the tested molecules (Z20 and Z91). Control 1/Control 2 molecules for CP450, LO, MP, NO and XO were FLU/S-warfarin, ZIL/protocatechuic acid, MEL/N-acetyl-D-glucosamine, DEX/adenosine-5’-diphosphate and FEB/hypoxanthine, respectively. Figure 4shows the interactions data of the tested (Z20 and Z91) and control (FLU) molecules with the CP450 receptor. Five amino acid residues (PRO367, ALA 103, LEU366, PHE 114 and ILE99) were common to Z20, Z91 and FLU (indicated by the blue in diagram A). This shows the degree of correspondence between the control and the molecules tested inferring that these may have potential antioxidant ability. The Z20 and Z91 molecules with lower number of interactions (seven and six, respectively) showed a BA ( − 7.8 kcal mol −1 and − 7.5 kcal mol −1 , respectively) higher than control 1 (FLU) with BA = − 9.8 kcal.mol −1 (eight interactions). It is possible to verify the tendency of the BA value to decrease on increasing the number of interactions. Molecules 2018,23, 2801 5 of 17 Additionally, study with CP450 structure complexed with S-warfarin shows that the active site of interaction is coated by ARG97, GLY98, ILE99, PHE100, LEU102, ALA103, VAL113, PHE114, ASN217, THR364, SER365, LEU366, PRO367 and PHE476 residues. Specific interactions occur between some of these residues and the phenyl group of S-warfarin (control 2), which bundles against the side chains of PHE476 (pi-pi interaction), PHE100 and ALA103 (hydrogen bond) and interact with PRO367 too [ 17 ]. Figure 4displays a high similarity of interactions with the CP450 receptor between the tested and control molecules, and findings of the literature [ 17 ]. These results for the tested molecules together to good BA values (differences of − 2.0 kcal mol −1 in Z20 and − 2.3 kcal mol −1 in Z91, in relation to the FLU control) indicate a good antioxidant ability to these molecules. Molecules 2018, 23, x 5 of 17 Additionally, study with CP450 structure complexed with S-warfarin shows that the active site of interaction is coated by ARG97, GLY98, ILE99, PHE100, LEU102, ALA103, VAL113, PHE114, ASN217, THR364, SER365, LEU366, PRO367 and PHE476 residues. Specific interactions occur between some of these residues and the phenyl group of S-warfarin (control 2), which bundles against the side chains of PHE476 (pi-pi interaction), PHE100 and ALA103 (hydrogen bond) and interact with PRO367 too [17]. Figure 4 displays a high similarity of interactions with the CP450 receptor between the tested and control molecules, and findings of the literature [17]. These results for the tested molecules together to good BA values (differences of −2.0 kcal mol −1 in Z20 and −2.3 kcal mol −1 in Z91, in relation to the FLU control) indicate a good antioxidant ability to these molecules. Figure 4. Interactions of the tested molecules (Z20 and Z91) and control (FLU) with the CP450 receptor. In A, interactions common to the three ligands (blue), two ligands (green) and interactions presented by a single ligand (yellow) are presented. Hydrogen bond (A); carbon-hydrogen bond (B); pi-sigma (C); pi-alkyl (D); alkyl(E). In the MP receptor (Figure 5), a maximum of six interactions were observed. In the PDB file (1DNU) [18] the active site of attachment has its location pointed out to the ASN192, GLN201 and VAL199 amino acid residues, which interact with the ligand N-acetyl-D-glucosamine. This ligand is complexed with the MP receptor available in the PDB (control 2, Figure 3). The three previously mentioned residues interact with the control molecule, MEL and two of them (GLN201 and VAL199) interact with Z20 and Z91, which indicates a reasonable antioxidant ability to the molecules tested. Other indications of good antioxidant ability are the similarities with the control molecule: Z20 and Z91 interact with four amino acid residues (blue in diagram A of Figure 5) common to the control and BA values lower than that of the control (−4.6 kcal mol −1 for Z20, −4.5 kcal mol −1 for Z91 and −3.6 kcal mol −1 for the MEL control). In NO (see Figure 6) the ASP179 residue only showed interaction with all molecules (blue in diagram A). For this receptor an increase in the number of interactions provided an increase in the BA value, when compared to Z91 (eight interactions and BA = −7.1 kcal.mol −1 ), and with DEX control (three interactions and BA = −8.4 kcal.mol −1 ). These BA values are relatively close (difference of −1,3 kcal mol −1 for Z20 and Z91) to the DEX control, which indicates a good antioxidant ability of the tested molecules in the NO receptor. Figure 4. Interactions of the tested molecules (Z20 and Z91) and control (FLU) with the CP450 receptor. In A, interactions common to the three ligands (blue), two ligands (green) and interactions presented by a single ligand (yellow) are presented. Molecules 2018, 23, x 5 of 17 Additionally, study with CP450 structure complexed with S-warfarin shows that the active site of interaction is coated by ARG97, GLY98, ILE99, PHE100, LEU102, ALA103, VAL113, PHE114, ASN217, THR364, SER365, LEU366, PRO367 and PHE476 residues. Specific interactions occur between some of these residues and the phenyl group of S-warfarin (control 2), which bundles against the side chains of PHE476 (pi-pi interaction), PHE100 and ALA103 (hydrogen bond) and interact with PRO367 too [17]. Figure 4 displays a high similarity of interactions with the CP450 receptor between the tested and control molecules, and findings of the literature [17]. These results for the tested molecules together to good BA values (differences of −2.0 kcal mol −1 in Z20 and −2.3 kcal mol −1 in Z91, in relation to the FLU control) indicate a good antioxidant ability to these molecules. Figure 4. Interactions of the tested molecules (Z20 and Z91) and control (FLU) with the CP450 receptor. In A, interactions common to the three ligands (blue), two ligands (green) and interactions presented by a single ligand (yellow) are presented. Hydrogen bond (A); carbon-hydrogen bond (B); pi-sigma (C); pi-alkyl (D); alkyl(E). In the MP receptor (Figure 5), a maximum of six interactions were observed. In the PDB file (1DNU) [18] the active site of attachment has its location pointed out to the ASN192, GLN201 and VAL199 amino acid residues, which interact with the ligand N-acetyl-D-glucosamine. This ligand is complexed with the MP receptor available in the PDB (control 2, Figure 3). The three previously mentioned residues interact with the control molecule, MEL and two of them (GLN201 and VAL199) interact with Z20 and Z91, which indicates a reasonable antioxidant ability to the molecules tested. Other indications of good antioxidant ability are the similarities with the control molecule: Z20 and Z91 interact with four amino acid residues (blue in diagram A of Figure 5) common to the control and BA values lower than that of the control (−4.6 kcal mol −1 for Z20, −4.5 kcal mol −1 for Z91 and −3.6 kcal mol −1 for the MEL control). In NO (see Figure 6) the ASP179 residue only showed interaction with all molecules (blue in diagram A). For this receptor an increase in the number of interactions provided an increase in the BA value, when compared to Z91 (eight interactions and BA = −7.1 kcal.mol −1 ), and with DEX control (three interactions and BA = −8.4 kcal.mol −1 ). These BA values are relatively close (difference of −1,3 kcal mol −1 for Z20 and Z91) to the DEX control, which indicates a good antioxidant ability of the tested molecules in the NO receptor. Hydrogen bond (A); Molecules 2018, 23, x 5 of 17 Additionally, study with CP450 structure complexed with S-warfarin shows that the active site of interaction is coated by ARG97, GLY98, ILE99, PHE100, LEU102, ALA103, VAL113, PHE114, ASN217, THR364, SER365, LEU366, PRO367 and PHE476 residues. Specific interactions occur between some of these residues and the phenyl group of S-warfarin (control 2), which bundles against the side chains of PHE476 (pi-pi interaction), PHE100 and ALA103 (hydrogen bond) and interact with PRO367 too [17]. Figure 4 displays a high similarity of interactions with the CP450 receptor between the tested and control molecules, and findings of the literature [17]. These results for the tested molecules together to good BA values (differences of −2.0 kcal mol −1 in Z20 and −2.3 kcal mol −1 in Z91, in relation to the FLU control) indicate a good antioxidant ability to these molecules. Figure 4. Interactions of the tested molecules (Z20 and Z91) and control (FLU) with the CP450 receptor. In A, interactions common to the three ligands (blue), two ligands (green) and interactions presented by a single ligand (yellow) are presented. Hydrogen bond (A); carbon-hydrogen bond (B); pi-sigma (C); pi-alkyl (D); alkyl(E). In the MP receptor (Figure 5), a maximum of six interactions were observed. In the PDB file (1DNU) [18] the active site of attachment has its location pointed out to the ASN192, GLN201 and VAL199 amino acid residues, which interact with the ligand N-acetyl-D-glucosamine. This ligand is complexed with the MP receptor available in the PDB (control 2, Figure 3). The three previously mentioned residues interact with the control molecule, MEL and two of them (GLN201 and VAL199) interact with Z20 and Z91, which indicates a reasonable antioxidant ability to the molecules tested. Other indications of good antioxidant ability are the similarities with the control molecule: Z20 and Z91 interact with four amino acid residues (blue in diagram A of Figure 5) common to the control and BA values lower than that of the control (−4.6 kcal mol −1 for Z20, −4.5 kcal mol −1 for Z91 and −3.6 kcal mol −1 for the MEL control). In NO (see Figure 6) the ASP179 residue only showed interaction with all molecules (blue in diagram A). For this receptor an increase in the number of interactions provided an increase in the BA value, when compared to Z91 (eight interactions and BA = −7.1 kcal.mol −1 ), and with DEX control (three interactions and BA = −8.4 kcal.mol −1 ). These BA values are relatively close (difference of −1,3 kcal mol −1 for Z20 and Z91) to the DEX control, which indicates a good antioxidant ability of the tested molecules in the NO receptor. carbon-hydrogen bond (B); Molecules 2018, 23, x 5 of 17 Additionally, study with CP450 structure complexed with S-warfarin shows that the active site of interaction is coated by ARG97, GLY98, ILE99, PHE100, LEU102, ALA103, VAL113, PHE114, ASN217, THR364, SER365, LEU366, PRO367 and PHE476 residues. Specific interactions occur between some of these residues and the phenyl group of S-warfarin (control 2), which bundles against the side chains of PHE476 (pi-pi interaction), PHE100 and ALA103 (hydrogen bond) and interact with PRO367 too [17]. Figure 4 displays a high similarity of interactions with the CP450 receptor between the tested and control molecules, and findings of the literature [17]. These results for the tested molecules together to good BA values (differences of −2.0 kcal mol −1 in Z20 and −2.3 kcal mol −1 in Z91, in relation to the FLU control) indicate a good antioxidant ability to these molecules. Figure 4. Interactions of the tested molecules (Z20 and Z91) and control (FLU) with the CP450 receptor. In A, interactions common to the three ligands (blue), two ligands (green) and interactions presented by a single ligand (yellow) are presented. Hydrogen bond (A); carbon-hydrogen bond (B); pi-sigma (C); pi-alkyl (D); alkyl(E). In the MP receptor (Figure 5), a maximum of six interactions were observed. In the PDB file (1DNU) [18] the active site of attachment has its location pointed out to the ASN192, GLN201 and VAL199 amino acid residues, which interact with the ligand N-acetyl-D-glucosamine. This ligand is complexed with the MP receptor available in the PDB (control 2, Figure 3). The three previously mentioned residues interact with the control molecule, MEL and two of them (GLN201 and VAL199) interact with Z20 and Z91, which indicates a reasonable antioxidant ability to the molecules tested. Other indications of good antioxidant ability are the similarities with the control molecule: Z20 and Z91 interact with four amino acid residues (blue in diagram A of Figure 5) common to the control and BA values lower than that of the control (−4.6 kcal mol −1 for Z20, −4.5 kcal mol −1 for Z91 and −3.6 kcal mol −1 for the MEL control). In NO (see Figure 6) the ASP179 residue only showed interaction with all molecules (blue in diagram A). For this receptor an increase in the number of interactions provided an increase in the BA value, when compared to Z91 (eight interactions and BA = −7.1 kcal.mol −1 ), and with DEX control (three interactions and BA = −8.4 kcal.mol −1 ). These BA values are relatively close (difference of −1,3 kcal mol −1 for Z20 and Z91) to the DEX control, which indicates a good antioxidant ability of the tested molecules in the NO receptor. pi-sigma (C); Molecules 2018, 23, x 5 of 17 Additionally, study with CP450 structure complexed with S-warfarin shows that the active site of interaction is coated by ARG97, GLY98, ILE99, PHE100, LEU102, ALA103, VAL113, PHE114, ASN217, THR364, SER365, LEU366, PRO367 and PHE476 residues. Specific interactions occur between some of these residues and the phenyl group of S-warfarin (control 2), which bundles against the side chains of PHE476 (pi-pi interaction), PHE100 and ALA103 (hydrogen bond) and interact with PRO367 too [17]. Figure 4 displays a high similarity of interactions with the CP450 receptor between the tested and control molecules, and findings of the literature [17]. These results for the tested molecules together to good BA values (differences of −2.0 kcal mol −1 in Z20 and −2.3 kcal mol −1 in Z91, in relation to the FLU control) indicate a good antioxidant ability to these molecules. Figure 4. Interactions of the tested molecules (Z20 and Z91) and control (FLU) with the CP450 receptor. In A, interactions common to the three ligands (blue), two ligands (green) and interactions presented by a single ligand (yellow) are presented. Hydrogen bond (A); carbon-hydrogen bond (B); pi-sigma (C); pi-alkyl (D); alkyl(E). In the MP receptor (Figure 5), a maximum of six interactions were observed. In the PDB file (1DNU) [18] the active site of attachment has its location pointed out to the ASN192, GLN201 and VAL199 amino acid residues, which interact with the ligand N-acetyl-D-glucosamine. This ligand is complexed with the MP receptor available in the PDB (control 2, Figure 3). The three previously mentioned residues interact with the control molecule, MEL and two of them (GLN201 and VAL199) interact with Z20 and Z91, which indicates a reasonable antioxidant ability to the molecules tested. Other indications of good antioxidant ability are the similarities with the control molecule: Z20 and Z91 interact with four amino acid residues (blue in diagram A of Figure 5) common to the control and BA values lower than that of the control (−4.6 kcal mol −1 for Z20, −4.5 kcal mol −1 for Z91 and −3.6 kcal mol −1 for the MEL control). In NO (see Figure 6) the ASP179 residue only showed interaction with all molecules (blue in diagram A). For this receptor an increase in the number of interactions provided an increase in the BA value, when compared to Z91 (eight interactions and BA = −7.1 kcal.mol −1 ), and with DEX control (three interactions and BA = −8.4 kcal.mol −1 ). These BA values are relatively close (difference of −1,3 kcal mol −1 for Z20 and Z91) to the DEX control, which indicates a good antioxidant ability of the tested molecules in the NO receptor. pi-alkyl (D); Molecules 2018, 23, x 5 of 17 Additionally, study with CP450 structure complexed with S-warfarin shows that the active site of interaction is coated by ARG97, GLY98, ILE99, PHE100, LEU102, ALA103, VAL113, PHE114, ASN217, THR364, SER365, LEU366, PRO367 and PHE476 residues. Specific interactions occur between some of these residues and the phenyl group of S-warfarin (control 2), which bundles against the side chains of PHE476 (pi-pi interaction), PHE100 and ALA103 (hydrogen bond) and interact with PRO367 too [17]. Figure 4 displays a high similarity of interactions with the CP450 receptor between the tested and control molecules, and findings of the literature [17]. These results for the tested molecules together to good BA values (differences of −2.0 kcal mol −1 in Z20 and −2.3 kcal mol −1 in Z91, in relation to the FLU control) indicate a good antioxidant ability to these molecules. Figure 4. Interactions of the tested molecules (Z20 and Z91) and control (FLU) with the CP450 receptor. In A, interactions common to the three ligands (blue), two ligands (green) and interactions presented by a single ligand (yellow) are presented. Hydrogen bond (A); carbon-hydrogen bond (B); pi-sigma (C); pi-alkyl (D); alkyl(E). In the MP receptor (Figure 5), a maximum of six interactions were observed. In the PDB file (1DNU) [18] the active site of attachment has its location pointed out to the ASN192, GLN201 and VAL199 amino acid residues, which interact with the ligand N-acetyl-D-glucosamine. This ligand is complexed with the MP receptor available in the PDB (control 2, Figure 3). The three previously mentioned residues interact with the control molecule, MEL and two of them (GLN201 and VAL199) interact with Z20 and Z91, which indicates a reasonable antioxidant ability to the molecules tested. Other indications of good antioxidant ability are the similarities with the control molecule: Z20 and Z91 interact with four amino acid residues (blue in diagram A of Figure 5) common to the control and BA values lower than that of the control (−4.6 kcal mol −1 for Z20, −4.5 kcal mol −1 for Z91 and −3.6 kcal mol −1 for the MEL control). In NO (see Figure 6) the ASP179 residue only showed interaction with all molecules (blue in diagram A). For this receptor an increase in the number of interactions provided an increase in the BA value, when compared to Z91 (eight interactions and BA = −7.1 kcal.mol −1 ), and with DEX control (three interactions and BA = −8.4 kcal.mol −1 ). These BA values are relatively close (difference of −1,3 kcal mol −1 for Z20 and Z91) to the DEX control, which indicates a good antioxidant ability of the tested molecules in the NO receptor. alkyl (E). In the MP receptor (Figure 5), a maximum of six interactions were observed. In the PDB file (1DNU) [ 18 ] the active site of attachment has its location pointed out to the ASN192, GLN201 and VAL199 amino acid residues, which interact with the ligand N-acetyl-D-glucosamine. This ligand is complexed with the MP receptor available in the PDB (control 2, Figure 3). The three previously mentioned residues interact with the control molecule, MEL and two of them (GLN201 and VAL199) interact with Z20 and Z91, which indicates a reasonable antioxidant ability to the molecules tested. Other indications of good antioxidant ability are the similarities with the control molecule: Z20 and Z91 interact with four amino acid residues (blue in diagram A of Figure 5) common to the control and BA values lower than that of the control ( − 4.6 kcal mol −1 for Z20, − 4.5 kcal mol −1 for Z91 and −3.6 kcal mol−1for the MEL control). In NO (see Figure 6) the ASP179 residue only showed interaction with all molecules (blue in diagram A). For this receptor an increase in the number of interactions provided an increase in the BA value, when compared to Z91 (eight interactions and BA = − 7.1 kcal.mol −1 ), and with DEX control (three interactions and BA = − 8.4 kcal.mol −1 ). These BA values are relatively close (difference of −1,3 kcal mol−1 for Z20 and Z91) to the DEX control, which indicates a good antioxidant ability of the tested molecules in the NO receptor. Molecules 2018,23, 2801 6 of 17 Another evidence of this antioxidant ability is the similarity of the interactions and of the active site with results obtained from the literature [ 19 ]. In such study, the active site is surrounded by ILE160, ILE243, ASP179, LYS213, VAL214 and TYR188 residues, which interact with the ligand adenosine-5’-diphosphate (Control 2 in the NO receptor—Figure 3). From the list of residues, four interact with Z91 (ILE243, ASP179, LYS213 and VAL214), three with Z20 (ILE243, ASP179 and TYR188) and one with the DEX control (ASP179). Molecules 2018, 23, x 6 of 17 Another evidence of this antioxidant ability is the similarity of the interactions and of the active site with results obtained from the literature [19]. In such study, the active site is surrounded by ILE160, ILE243, ASP179, LYS213, VAL214 and TYR188 residues, which interact with the ligand adenosine-5’-diphosphate (Control 2 in the NO receptor—Figure 3). From the list of residues, four interact with Z91 (ILE243, ASP179, LYS213 and VAL214), three with Z20 (ILE243, ASP179 and TYR188) and one with the DEX control (ASP179). Figure 5. Interactions of the tested molecules (Z20 and Z91) and control (MEL) with the MP receptor. In A, interactions common to the three ligands (blue) and two ligands (green). Hydrogen bond (A); carbon-hydrogen bond (B); pi-sigma (C); pi-alkyl (D); alkyl(E). Figure 6. Interactions of the tested molecules (Z20 and Z91) and control (DEX) with the NO receptor. In A, interactions common to the three ligands (blue), two ligands (green) and interactions presented by a single ligand (yellow) are presented. Hydrogen bond (A); carbon-hydrogen bond (B); pi-sigma (C); pi-alkyl (D); akyl(E). Figure 5. Interactions of the tested molecules (Z20 and Z91) and control (MEL) with the MP receptor. In A, interactions common to the three ligands (blue) and two ligands (green). Molecules 2018, 23, x 5 of 17 Additionally, study with CP450 structure complexed with S-warfarin shows that the active site of interaction is coated by ARG97, GLY98, ILE99, PHE100, LEU102, ALA103, VAL113, PHE114, ASN217, THR364, SER365, LEU366, PRO367 and PHE476 residues. Specific interactions occur between some of these residues and the phenyl group of S-warfarin (control 2), which bundles against the side chains of PHE476 (pi-pi interaction), PHE100 and ALA103 (hydrogen bond) and interact with PRO367 too [17]. Figure 4 displays a high similarity of interactions with the CP450 receptor between the tested and control molecules, and findings of the literature [17]. These results for the tested molecules together to good BA values (differences of −2.0 kcal mol −1 in Z20 and −2.3 kcal mol −1 in Z91, in relation to the FLU control) indicate a good antioxidant ability to these molecules. Figure 4. Interactions of the tested molecules (Z20 and Z91) and control (FLU) with the CP450 receptor. In A, interactions common to the three ligands (blue), two ligands (green) and interactions presented by a single ligand (yellow) are presented. Hydrogen bond (A); carbon-hydrogen bond (B); pi-sigma (C); pi-alkyl (D); alkyl(E). In the MP receptor (Figure 5), a maximum of six interactions were observed. In the PDB file (1DNU) [18] the active site of attachment has its location pointed out to the ASN192, GLN201 and VAL199 amino acid residues, which interact with the ligand N-acetyl-D-glucosamine. This ligand is complexed with the MP receptor available in the PDB (control 2, Figure 3). The three previously mentioned residues interact with the control molecule, MEL and two of them (GLN201 and VAL199) interact with Z20 and Z91, which indicates a reasonable antioxidant ability to the molecules tested. Other indications of good antioxidant ability are the similarities with the control molecule: Z20 and Z91 interact with four amino acid residues (blue in diagram A of Figure 5) common to the control and BA values lower than that of the control (−4.6 kcal mol −1 for Z20, −4.5 kcal mol −1 for Z91 and −3.6 kcal mol −1 for the MEL control). In NO (see Figure 6) the ASP179 residue only showed interaction with all molecules (blue in diagram A). For this receptor an increase in the number of interactions provided an increase in the BA value, when compared to Z91 (eight interactions and BA = −7.1 kcal.mol −1 ), and with DEX control (three interactions and BA = −8.4 kcal.mol −1 ). These BA values are relatively close (difference of −1,3 kcal mol −1 for Z20 and Z91) to the DEX control, which indicates a good antioxidant ability of the tested molecules in the NO receptor. Hydrogen bond (A); Molecules 2018, 23, x 5 of 17 Additionally, study with CP450 structure complexed with S-warfarin shows that the active site of interaction is coated by ARG97, GLY98, ILE99, PHE100, LEU102, ALA103, VAL113, PHE114, ASN217, THR364, SER365, LEU366, PRO367 and PHE476 residues. Specific interactions occur between some of these residues and the phenyl group of S-warfarin (control 2), which bundles against the side chains of PHE476 (pi-pi interaction), PHE100 and ALA103 (hydrogen bond) and interact with PRO367 too [17]. Figure 4 displays a high similarity of interactions with the CP450 receptor between the tested and control molecules, and findings of the literature [17]. These results for the tested molecules together to good BA values (differences of −2.0 kcal mol −1 in Z20 and −2.3 kcal mol −1 in Z91, in relation to the FLU control) indicate a good antioxidant ability to these molecules. Figure 4. Interactions of the tested molecules (Z20 and Z91) and control (FLU) with the CP450 receptor. In A, interactions common to the three ligands (blue), two ligands (green) and interactions presented by a single ligand (yellow) are presented. Hydrogen bond (A); carbon-hydrogen bond (B); pi-sigma (C); pi-alkyl (D); alkyl(E). In the MP receptor (Figure 5), a maximum of six interactions were observed. In the PDB file (1DNU) [18] the active site of attachment has its location pointed out to the ASN192, GLN201 and VAL199 amino acid residues, which interact with the ligand N-acetyl-D-glucosamine. This ligand is complexed with the MP receptor available in the PDB (control 2, Figure 3). The three previously mentioned residues interact with the control molecule, MEL and two of them (GLN201 and VAL199) interact with Z20 and Z91, which indicates a reasonable antioxidant ability to the molecules tested. Other indications of good antioxidant ability are the similarities with the control molecule: Z20 and Z91 interact with four amino acid residues (blue in diagram A of Figure 5) common to the control and BA values lower than that of the control (−4.6 kcal mol −1 for Z20, −4.5 kcal mol −1 for Z91 and −3.6 kcal mol −1 for the MEL control). In NO (see Figure 6) the ASP179 residue only showed interaction with all molecules (blue in diagram A). For this receptor an increase in the number of interactions provided an increase in the BA value, when compared to Z91 (eight interactions and BA = −7.1 kcal.mol −1 ), and with DEX control (three interactions and BA = −8.4 kcal.mol −1 ). These BA values are relatively close (difference of −1,3 kcal mol −1 for Z20 and Z91) to the DEX control, which indicates a good antioxidant ability of the tested molecules in the NO receptor. carbon-hydrogen bond (B); Molecules 2018, 23, x 5 of 17 Additionally, study with CP450 structure complexed with S-warfarin shows that the active site of interaction is coated by ARG97, GLY98, ILE99, PHE100, LEU102, ALA103, VAL113, PHE114, ASN217, THR364, SER365, LEU366, PRO367 and PHE476 residues. Specific interactions occur between some of these residues and the phenyl group of S-warfarin (control 2), which bundles against the side chains of PHE476 (pi-pi interaction), PHE100 and ALA103 (hydrogen bond) and interact with PRO367 too [17]. Figure 4 displays a high similarity of interactions with the CP450 receptor between the tested and control molecules, and findings of the literature [17]. These results for the tested molecules together to good BA values (differences of −2.0 kcal mol −1 in Z20 and −2.3 kcal mol −1 in Z91, in relation to the FLU control) indicate a good antioxidant ability to these molecules. Figure 4. Interactions of the tested molecules (Z20 and Z91) and control (FLU) with the CP450 receptor. In A, interactions common to the three ligands (blue), two ligands (green) and interactions presented by a single ligand (yellow) are presented. Hydrogen bond (A); carbon-hydrogen bond (B); pi-sigma (C); pi-alkyl (D); alkyl(E). In the MP receptor (Figure 5), a maximum of six interactions were observed. In the PDB file (1DNU) [18] the active site of attachment has its location pointed out to the ASN192, GLN201 and VAL199 amino acid residues, which interact with the ligand N-acetyl-D-glucosamine. This ligand is complexed with the MP receptor available in the PDB (control 2, Figure 3). The three previously mentioned residues interact with the control molecule, MEL and two of them (GLN201 and VAL199) interact with Z20 and Z91, which indicates a reasonable antioxidant ability to the molecules tested. Other indications of good antioxidant ability are the similarities with the control molecule: Z20 and Z91 interact with four amino acid residues (blue in diagram A of Figure 5) common to the control and BA values lower than that of the control (−4.6 kcal mol −1 for Z20, −4.5 kcal mol −1 for Z91 and −3.6 kcal mol −1 for the MEL control). In NO (see Figure 6) the ASP179 residue only showed interaction with all molecules (blue in diagram A). For this receptor an increase in the number of interactions provided an increase in the BA value, when compared to Z91 (eight interactions and BA = −7.1 kcal.mol −1 ), and with DEX control (three interactions and BA = −8.4 kcal.mol −1 ). These BA values are relatively close (difference of −1,3 kcal mol −1 for Z20 and Z91) to the DEX control, which indicates a good antioxidant ability of the tested molecules in the NO receptor. pi-sigma (C); Molecules 2018, 23, x 5 of 17 Additionally, study with CP450 structure complexed with S-warfarin shows that the active site of interaction is coated by ARG97, GLY98, ILE99, PHE100, LEU102, ALA103, VAL113, PHE114, ASN217, THR364, SER365, LEU366, PRO367 and PHE476 residues. Specific interactions occur between some of these residues and the phenyl group of S-warfarin (control 2), which bundles against the side chains of PHE476 (pi-pi interaction), PHE100 and ALA103 (hydrogen bond) and interact with PRO367 too [17]. Figure 4 displays a high similarity of interactions with the CP450 receptor between the tested and control molecules, and findings of the literature [17]. These results for the tested molecules together to good BA values (differences of −2.0 kcal mol −1 in Z20 and −2.3 kcal mol −1 in Z91, in relation to the FLU control) indicate a good antioxidant ability to these molecules. Figure 4. Interactions of the tested molecules (Z20 and Z91) and control (FLU) with the CP450 receptor. In A, interactions common to the three ligands (blue), two ligands (green) and interactions presented by a single ligand (yellow) are presented. Hydrogen bond (A); carbon-hydrogen bond (B); pi-sigma (C); pi-alkyl (D); alkyl(E). In the MP receptor (Figure 5), a maximum of six interactions were observed. In the PDB file (1DNU) [18] the active site of attachment has its location pointed out to the ASN192, GLN201 and VAL199 amino acid residues, which interact with the ligand N-acetyl-D-glucosamine. This ligand is complexed with the MP receptor available in the PDB (control 2, Figure 3). The three previously mentioned residues interact with the control molecule, MEL and two of them (GLN201 and VAL199) interact with Z20 and Z91, which indicates a reasonable antioxidant ability to the molecules tested. Other indications of good antioxidant ability are the similarities with the control molecule: Z20 and Z91 interact with four amino acid residues (blue in diagram A of Figure 5) common to the control and BA values lower than that of the control (−4.6 kcal mol −1 for Z20, −4.5 kcal mol −1 for Z91 and −3.6 kcal mol −1 for the MEL control). In NO (see Figure 6) the ASP179 residue only showed interaction with all molecules (blue in diagram A). For this receptor an increase in the number of interactions provided an increase in the BA value, when compared to Z91 (eight interactions and BA = −7.1 kcal.mol −1 ), and with DEX control (three interactions and BA = −8.4 kcal.mol −1 ). These BA values are relatively close (difference of −1,3 kcal mol −1 for Z20 and Z91) to the DEX control, which indicates a good antioxidant ability of the tested molecules in the NO receptor. pi-alkyl (D); Molecules 2018, 23, x 5 of 17 Additionally, study with CP450 structure complexed with S-warfarin shows that the active site of interaction is coated by ARG97, GLY98, ILE99, PHE100, LEU102, ALA103, VAL113, PHE114, ASN217, THR364, SER365, LEU366, PRO367 and PHE476 residues. Specific interactions occur between some of these residues and the phenyl group of S-warfarin (control 2), which bundles against the side chains of PHE476 (pi-pi interaction), PHE100 and ALA103 (hydrogen bond) and interact with PRO367 too [17]. Figure 4 displays a high similarity of interactions with the CP450 receptor between the tested and control molecules, and findings of the literature [17]. These results for the tested molecules together to good BA values (differences of −2.0 kcal mol −1 in Z20 and −2.3 kcal mol −1 in Z91, in relation to the FLU control) indicate a good antioxidant ability to these molecules. Figure 4. Interactions of the tested molecules (Z20 and Z91) and control (FLU) with the CP450 receptor. In A, interactions common to the three ligands (blue), two ligands (green) and interactions presented by a single ligand (yellow) are presented. Hydrogen bond (A); carbon-hydrogen bond (B); pi-sigma (C); pi-alkyl (D); alkyl(E). In the MP receptor (Figure 5), a maximum of six interactions were observed. In the PDB file (1DNU) [18] the active site of attachment has its location pointed out to the ASN192, GLN201 and VAL199 amino acid residues, which interact with the ligand N-acetyl-D-glucosamine. This ligand is complexed with the MP receptor available in the PDB (control 2, Figure 3). The three previously mentioned residues interact with the control molecule, MEL and two of them (GLN201 and VAL199) interact with Z20 and Z91, which indicates a reasonable antioxidant ability to the molecules tested. Other indications of good antioxidant ability are the similarities with the control molecule: Z20 and Z91 interact with four amino acid residues (blue in diagram A of Figure 5) common to the control and BA values lower than that of the control (−4.6 kcal mol −1 for Z20, −4.5 kcal mol −1 for Z91 and −3.6 kcal mol −1 for the MEL control). In NO (see Figure 6) the ASP179 residue only showed interaction with all molecules (blue in diagram A). For this receptor an increase in the number of interactions provided an increase in the BA value, when compared to Z91 (eight interactions and BA = −7.1 kcal.mol −1 ), and with DEX control (three interactions and BA = −8.4 kcal.mol −1 ). These BA values are relatively close (difference of −1,3 kcal mol −1 for Z20 and Z91) to the DEX control, which indicates a good antioxidant ability of the tested molecules in the NO receptor. alkyl (E). Molecules 2018, 23, x 6 of 17 Another evidence of this antioxidant ability is the similarity of the interactions and of the active site with results obtained from the literature [19]. In such study, the active site is surrounded by ILE160, ILE243, ASP179, LYS213, VAL214 and TYR188 residues, which interact with the ligand adenosine-5’-diphosphate (Control 2 in the NO receptor—Figure 3). From the list of residues, four interact with Z91 (ILE243, ASP179, LYS213 and VAL214), three with Z20 (ILE243, ASP179 and TYR188) and one with the DEX control (ASP179). Figure 5. Interactions of the tested molecules (Z20 and Z91) and control (MEL) with the MP receptor. In A, interactions common to the three ligands (blue) and two ligands (green). Hydrogen bond (A); carbon-hydrogen bond (B); pi-sigma (C); pi-alkyl (D); alkyl(E). Figure 6. Interactions of the tested molecules (Z20 and Z91) and control (DEX) with the NO receptor. In A, interactions common to the three ligands (blue), two ligands (green) and interactions presented by a single ligand (yellow) are presented. Hydrogen bond (A); carbon-hydrogen bond (B); pi-sigma (C); pi-alkyl (D); akyl(E). Figure 6. Interactions of the tested molecules (Z20 and Z91) and control (DEX) with the NO receptor. In A, interactions common to the three ligands (blue), two ligands (green) and interactions presented by a single ligand (yellow) are presented. Molecules 2018, 23, x 5 of 17 Additionally, study with CP450 structure complexed with S-warfarin shows that the active site of interaction is coated by ARG97, GLY98, ILE99, PHE100, LEU102, ALA103, VAL113, PHE114, ASN217, THR364, SER365, LEU366, PRO367 and PHE476 residues. Specific interactions occur between some of these residues and the phenyl group of S-warfarin (control 2), which bundles against the side chains of PHE476 (pi-pi interaction), PHE100 and ALA103 (hydrogen bond) and interact with PRO367 too [17]. Figure 4 displays a high similarity of interactions with the CP450 receptor between the tested and control molecules, and findings of the literature [17]. These results for the tested molecules together to good BA values (differences of −2.0 kcal mol −1 in Z20 and −2.3 kcal mol −1 in Z91, in relation to the FLU control) indicate a good antioxidant ability to these molecules. Figure 4. Interactions of the tested molecules (Z20 and Z91) and control (FLU) with the CP450 receptor. In A, interactions common to the three ligands (blue), two ligands (green) and interactions presented by a single ligand (yellow) are presented. Hydrogen bond (A); carbon-hydrogen bond (B); pi-sigma (C); pi-alkyl (D); alkyl(E). In the MP receptor (Figure 5), a maximum of six interactions were observed. In the PDB file (1DNU) [18] the active site of attachment has its location pointed out to the ASN192, GLN201 and VAL199 amino acid residues, which interact with the ligand N-acetyl-D-glucosamine. This ligand is complexed with the MP receptor available in the PDB (control 2, Figure 3). The three previously mentioned residues interact with the control molecule, MEL and two of them (GLN201 and VAL199) interact with Z20 and Z91, which indicates a reasonable antioxidant ability to the molecules tested. Other indications of good antioxidant ability are the similarities with the control molecule: Z20 and Z91 interact with four amino acid residues (blue in diagram A of Figure 5) common to the control and BA values lower than that of the control (−4.6 kcal mol −1 for Z20, −4.5 kcal mol −1 for Z91 and −3.6 kcal mol −1 for the MEL control). In NO (see Figure 6) the ASP179 residue only showed interaction with all molecules (blue in diagram A). For this receptor an increase in the number of interactions provided an increase in the BA value, when compared to Z91 (eight interactions and BA = −7.1 kcal.mol −1 ), and with DEX control (three interactions and BA = −8.4 kcal.mol −1 ). These BA values are relatively close (difference of −1,3 kcal mol −1 for Z20 and Z91) to the DEX control, which indicates a good antioxidant ability of the tested molecules in the NO receptor. Hydrogen bond (A); Molecules 2018, 23, x 5 of 17 Additionally, study with CP450 structure complexed with S-warfarin shows that the active site of interaction is coated by ARG97, GLY98, ILE99, PHE100, LEU102, ALA103, VAL113, PHE114, ASN217, THR364, SER365, LEU366, PRO367 and PHE476 residues. Specific interactions occur between some of these residues and the phenyl group of S-warfarin (control 2), which bundles against the side chains of PHE476 (pi-pi interaction), PHE100 and ALA103 (hydrogen bond) and interact with PRO367 too [17]. Figure 4 displays a high similarity of interactions with the CP450 receptor between the tested and control molecules, and findings of the literature [17]. These results for the tested molecules together to good BA values (differences of −2.0 kcal mol −1 in Z20 and −2.3 kcal mol −1 in Z91, in relation to the FLU control) indicate a good antioxidant ability to these molecules. Figure 4. Interactions of the tested molecules (Z20 and Z91) and control (FLU) with the CP450 receptor. In A, interactions common to the three ligands (blue), two ligands (green) and interactions presented by a single ligand (yellow) are presented. Hydrogen bond (A); carbon-hydrogen bond (B); pi-sigma (C); pi-alkyl (D); alkyl(E). In the MP receptor (Figure 5), a maximum of six interactions were observed. In the PDB file (1DNU) [18] the active site of attachment has its location pointed out to the ASN192, GLN201 and VAL199 amino acid residues, which interact with the ligand N-acetyl-D-glucosamine. This ligand is complexed with the MP receptor available in the PDB (control 2, Figure 3). The three previously mentioned residues interact with the control molecule, MEL and two of them (GLN201 and VAL199) interact with Z20 and Z91, which indicates a reasonable antioxidant ability to the molecules tested. Other indications of good antioxidant ability are the similarities with the control molecule: Z20 and Z91 interact with four amino acid residues (blue in diagram A of Figure 5) common to the control and BA values lower than that of the control (−4.6 kcal mol −1 for Z20, −4.5 kcal mol −1 for Z91 and −3.6 kcal mol −1 for the MEL control). In NO (see Figure 6) the ASP179 residue only showed interaction with all molecules (blue in diagram A). For this receptor an increase in the number of interactions provided an increase in the BA value, when compared to Z91 (eight interactions and BA = −7.1 kcal.mol −1 ), and with DEX control (three interactions and BA = −8.4 kcal.mol −1 ). These BA values are relatively close (difference of −1,3 kcal mol −1 for Z20 and Z91) to the DEX control, which indicates a good antioxidant ability of the tested molecules in the NO receptor. carbon-hydrogen bond (B); Molecules 2018, 23, x 5 of 17 Additionally, study with CP450 structure complexed with S-warfarin shows that the active site of interaction is coated by ARG97, GLY98, ILE99, PHE100, LEU102, ALA103, VAL113, PHE114, ASN217, THR364, SER365, LEU366, PRO367 and PHE476 residues. Specific interactions occur between some of these residues and the phenyl group of S-warfarin (control 2), which bundles against the side chains of PHE476 (pi-pi interaction), PHE100 and ALA103 (hydrogen bond) and interact with PRO367 too [17]. Figure 4 displays a high similarity of interactions with the CP450 receptor between the tested and control molecules, and findings of the literature [17]. These results for the tested molecules together to good BA values (differences of −2.0 kcal mol −1 in Z20 and −2.3 kcal mol −1 in Z91, in relation to the FLU control) indicate a good antioxidant ability to these molecules. Figure 4. Interactions of the tested molecules (Z20 and Z91) and control (FLU) with the CP450 receptor. In A, interactions common to the three ligands (blue), two ligands (green) and interactions presented by a single ligand (yellow) are presented. Hydrogen bond (A); carbon-hydrogen bond (B); pi-sigma (C); pi-alkyl (D); alkyl(E). In the MP receptor (Figure 5), a maximum of six interactions were observed. In the PDB file (1DNU) [18] the active site of attachment has its location pointed out to the ASN192, GLN201 and VAL199 amino acid residues, which interact with the ligand N-acetyl-D-glucosamine. This ligand is complexed with the MP receptor available in the PDB (control 2, Figure 3). The three previously mentioned residues interact with the control molecule, MEL and two of them (GLN201 and VAL199) interact with Z20 and Z91, which indicates a reasonable antioxidant ability to the molecules tested. Other indications of good antioxidant ability are the similarities with the control molecule: Z20 and Z91 interact with four amino acid residues (blue in diagram A of Figure 5) common to the control and BA values lower than that of the control (−4.6 kcal mol −1 for Z20, −4.5 kcal mol −1 for Z91 and −3.6 kcal mol −1 for the MEL control). In NO (see Figure 6) the ASP179 residue only showed interaction with all molecules (blue in diagram A). For this receptor an increase in the number of interactions provided an increase in the BA value, when compared to Z91 (eight interactions and BA = −7.1 kcal.mol −1 ), and with DEX control (three interactions and BA = −8.4 kcal.mol −1 ). These BA values are relatively close (difference of −1,3 kcal mol −1 for Z20 and Z91) to the DEX control, which indicates a good antioxidant ability of the tested molecules in the NO receptor. pi-sigma (C); Molecules 2018, 23, x 5 of 17 Additionally, study with CP450 structure complexed with S-warfarin shows that the active site of interaction is coated by ARG97, GLY98, ILE99, PHE100, LEU102, ALA103, VAL113, PHE114, ASN217, THR364, SER365, LEU366, PRO367 and PHE476 residues. Specific interactions occur between some of these residues and the phenyl group of S-warfarin (control 2), which bundles against the side chains of PHE476 (pi-pi interaction), PHE100 and ALA103 (hydrogen bond) and interact with PRO367 too [17]. Figure 4 displays a high similarity of interactions with the CP450 receptor between the tested and control molecules, and findings of the literature [17]. These results for the tested molecules together to good BA values (differences of −2.0 kcal mol −1 in Z20 and −2.3 kcal mol −1 in Z91, in relation to the FLU control) indicate a good antioxidant ability to these molecules. Figure 4. Interactions of the tested molecules (Z20 and Z91) and control (FLU) with the CP450 receptor. In A, interactions common to the three ligands (blue), two ligands (green) and interactions presented by a single ligand (yellow) are presented. Hydrogen bond (A); carbon-hydrogen bond (B); pi-sigma (C); pi-alkyl (D); alkyl(E). In the MP receptor (Figure 5), a maximum of six interactions were observed. In the PDB file (1DNU) [18] the active site of attachment has its location pointed out to the ASN192, GLN201 and VAL199 amino acid residues, which interact with the ligand N-acetyl-D-glucosamine. This ligand is complexed with the MP receptor available in the PDB (control 2, Figure 3). The three previously mentioned residues interact with the control molecule, MEL and two of them (GLN201 and VAL199) interact with Z20 and Z91, which indicates a reasonable antioxidant ability to the molecules tested. Other indications of good antioxidant ability are the similarities with the control molecule: Z20 and Z91 interact with four amino acid residues (blue in diagram A of Figure 5) common to the control and BA values lower than that of the control (−4.6 kcal mol −1 for Z20, −4.5 kcal mol −1 for Z91 and −3.6 kcal mol −1 for the MEL control). In NO (see Figure 6) the ASP179 residue only showed interaction with all molecules (blue in diagram A). For this receptor an increase in the number of interactions provided an increase in the BA value, when compared to Z91 (eight interactions and BA = −7.1 kcal.mol −1 ), and with DEX control (three interactions and BA = −8.4 kcal.mol −1 ). These BA values are relatively close (difference of −1,3 kcal mol −1 for Z20 and Z91) to the DEX control, which indicates a good antioxidant ability of the tested molecules in the NO receptor. pi-alkyl (D); Molecules 2018, 23, x 5 of 17 Additionally, study with CP450 structure complexed with S-warfarin shows that the active site of interaction is coated by ARG97, GLY98, ILE99, PHE100, LEU102, ALA103, VAL113, PHE114, ASN217, THR364, SER365, LEU366, PRO367 and PHE476 residues. Specific interactions occur between some of these residues and the phenyl group of S-warfarin (control 2), which bundles against the side chains of PHE476 (pi-pi interaction), PHE100 and ALA103 (hydrogen bond) and interact with PRO367 too [17]. Figure 4 displays a high similarity of interactions with the CP450 receptor between the tested and control molecules, and findings of the literature [17]. These results for the tested molecules together to good BA values (differences of −2.0 kcal mol −1 in Z20 and −2.3 kcal mol −1 in Z91, in relation to the FLU control) indicate a good antioxidant ability to these molecules. Figure 4. Interactions of the tested molecules (Z20 and Z91) and control (FLU) with the CP450 receptor. In A, interactions common to the three ligands (blue), two ligands (green) and interactions presented by a single ligand (yellow) are presented. Hydrogen bond (A); carbon-hydrogen bond (B); pi-sigma (C); pi-alkyl (D); alkyl(E). In the MP receptor (Figure 5), a maximum of six interactions were observed. In the PDB file (1DNU) [18] the active site of attachment has its location pointed out to the ASN192, GLN201 and VAL199 amino acid residues, which interact with the ligand N-acetyl-D-glucosamine. This ligand is complexed with the MP receptor available in the PDB (control 2, Figure 3). The three previously mentioned residues interact with the control molecule, MEL and two of them (GLN201 and VAL199) interact with Z20 and Z91, which indicates a reasonable antioxidant ability to the molecules tested. Other indications of good antioxidant ability are the similarities with the control molecule: Z20 and Z91 interact with four amino acid residues (blue in diagram A of Figure 5) common to the control and BA values lower than that of the control (−4.6 kcal mol −1 for Z20, −4.5 kcal mol −1 for Z91 and −3.6 kcal mol −1 for the MEL control). In NO (see Figure 6) the ASP179 residue only showed interaction with all molecules (blue in diagram A). For this receptor an increase in the number of interactions provided an increase in the BA value, when compared to Z91 (eight interactions and BA = −7.1 kcal.mol −1 ), and with DEX control (three interactions and BA = −8.4 kcal.mol −1 ). These BA values are relatively close (difference of −1,3 kcal mol −1 for Z20 and Z91) to the DEX control, which indicates a good antioxidant ability of the tested molecules in the NO receptor. akyl (E). Molecules 2018,23, 2801 7 of 17 The antioxidant abilities of the control molecules may be related to the amino acid residues with which molecules interact. For this reason, the observation of the common residues and BA values close to the tested molecules, controls and literature infer that the tested molecules can have a good antioxidant ability. This relationship is very evident for both Z20 and Z91 that have high similarity in the presented characteristics (among themselves, compared to controls and BA values). This can be explained by the high structural similarity of Z20 and Z91, with evident difference only in the four-carbon radical connected to nitrogen, showing that the alkenyl group (Z91) and the vinyl group are attached to the second carbon of radical (Z20, see Table 1). Table 1. Structure of the tested molecules used for in silico evaluation of the antioxidant potential. Molecule Assignment Molecules 2018, 23, x 7 of 17 The antioxidant abilities of the control molecules may be related to the amino acid residues with which molecules interact. For this reason, the observation of the common residues and BA values close to the tested molecules, controls and literature infer that the tested molecules can have a good antioxidant ability. This relationship is very evident for both Z20 and Z91 that have high similarity in the presented characteristics (among themselves, compared to controls and BA values). This can be explained by the high structural similarity of Z20 and Z91, with evident difference only in the four-carbon radical connected to nitrogen, showing that the alkenyl group (Z91) and the vinyl group are attached to the second carbon of radical (Z20, see Table 1). Table 1. Structure of the tested molecules used for in silico evaluation of the antioxidant potential. Molecule Assignment CH3 CH3 CH3 CH3 N N N N N O O ZINC08706191 (Z91) Tested molecule CH2 CH3 CH3 CH3 CH3 N N N N N O O ZINC08992920 (Z20) Tested molecule 2.2. Molecular Descriptors and Pearson Correlations Table 2 shows the data of the molecular descriptors and correlations with the Ki values for the three receptors analyzed (CP450, MP and NO). Ki (inhibition constant) is the concentration of the inhibitor needed to reduce the activity of the receptor by half. It reflects the binding affinity of the inhibitor with a specific receptor. The lower Ki value, the smaller the amount of inhibitor needed to reduce the reaction rate (inhibit the reaction) and the better the binding affinity [20]. The tested molecules Z20 and Z91 presented lower Ki values than the control molecules, which proves good antioxidant abilities for the tested molecules. The molecular descriptors presented reasonable results of correlations with the Ki values (between 0.65 and 0.99) for all descriptors in the CP450 receptor. Already the NO receptor presented four significant correlations (between 0.48 and 0.74), while the MP receptor had two significant correlations (both 0.60). Relatively low correlation values may be considered non-significant according to the literature [21,22]. This shows that some of these descriptors have significant relation (values in bold) with the Ki values, (Table 2), reaffirming again the notable antioxidant ability. Total surface area (TSA) of a molecule is related to its solubility and the molecular interaction that it can performed due to the superficial contact with other molecules [23]. Similar analysis can be done for the molar volume (MV), because the higher the TSA and MV, the higher the level of interaction. From TSA and MV the intermolecular forces operate, giving rise to the energy of molecular interaction [24]. The Ki values showed significant and negative correlations with TSA and MV only in CP450 and NO, indicating that the increase in these descriptors values is important for the Ki to decrease (increased antioxidant ability) in the mentioned receptors. Molecules formed by different atoms have a polar covalent bond that can be quantified by a dipole moment. This is described with a positive partial charge close to a negative partial charge of the same absolute value, and the measure of the magnitude of these charges is given in Debyes (D). In the case of polyatomic molecules, a total dipole moment (TDM) is observed. The higher the TDM, the greater the polarity of the molecule and consequently greater a tendency to dissolve and interact Tested molecule Molecules 2018, 23, x 7 of 17 The antioxidant abilities of the control molecules may be related to the amino acid residues with which molecules interact. For this reason, the observation of the common residues and BA values close to the tested molecules, controls and literature infer that the tested molecules can have a good antioxidant ability. This relationship is very evident for both Z20 and Z91 that have high similarity in the presented characteristics (among themselves, compared to controls and BA values). This can be explained by the high structural similarity of Z20 and Z91, with evident difference only in the four-carbon radical connected to nitrogen, showing that the alkenyl group (Z91) and the vinyl group are attached to the second carbon of radical (Z20, see Table 1). Table 1. Structure of the tested molecules used for in silico evaluation of the antioxidant potential. Molecule Assignment CH3 CH3 CH3 CH3 N N N N N O O ZINC08706191 (Z91) Tested molecule CH2 CH3 CH3 CH3 CH3 N N N N N O O ZINC08992920 (Z20) Tested molecule 2.2. Molecular Descriptors and Pearson Correlations Table 2 shows the data of the molecular descriptors and correlations with the Ki values for the three receptors analyzed (CP450, MP and NO). Ki (inhibition constant) is the concentration of the inhibitor needed to reduce the activity of the receptor by half. It reflects the binding affinity of the inhibitor with a specific receptor. The lower Ki value, the smaller the amount of inhibitor needed to reduce the reaction rate (inhibit the reaction) and the better the binding affinity [20]. The tested molecules Z20 and Z91 presented lower Ki values than the control molecules, which proves good antioxidant abilities for the tested molecules. The molecular descriptors presented reasonable results of correlations with the Ki values (between 0.65 and 0.99) for all descriptors in the CP450 receptor. Already the NO receptor presented four significant correlations (between 0.48 and 0.74), while the MP receptor had two significant correlations (both 0.60). Relatively low correlation values may be considered non-significant according to the literature [21,22]. This shows that some of these descriptors have significant relation (values in bold) with the Ki values, (Table 2), reaffirming again the notable antioxidant ability. Total surface area (TSA) of a molecule is related to its solubility and the molecular interaction that it can performed due to the superficial contact with other molecules [23]. Similar analysis can be done for the molar volume (MV), because the higher the TSA and MV, the higher the level of interaction. From TSA and MV the intermolecular forces operate, giving rise to the energy of molecular interaction [24]. The Ki values showed significant and negative correlations with TSA and MV only in CP450 and NO, indicating that the increase in these descriptors values is important for the Ki to decrease (increased antioxidant ability) in the mentioned receptors. Molecules formed by different atoms have a polar covalent bond that can be quantified by a dipole moment. This is described with a positive partial charge close to a negative partial charge of the same absolute value, and the measure of the magnitude of these charges is given in Debyes (D). In the case of polyatomic molecules, a total dipole moment (TDM) is observed. The higher the TDM, the greater the polarity of the molecule and consequently greater a tendency to dissolve and interact Tested molecule 2.2. Molecular Descriptors and Pearson Correlations Table 2shows the data of the molecular descriptors and correlations with the Ki values for the three receptors analyzed (CP450, MP and NO). Ki (inhibition constant) is the concentration of the inhibitor needed to reduce the activity of the receptor by half. It reflects the binding affinity of the inhibitor with a specific receptor. The lower Ki value, the smaller the amount of inhibitor needed to reduce the reaction rate (inhibit the reaction) and the better the binding affinity [ 20 ]. The tested molecules Z20 and Z91 presented lower Ki values than the control molecules, which proves good antioxidant abilities for the tested molecules. The molecular descriptors presented reasonable results of correlations with the Ki values (between 0.65 and 0.99) for all descriptors in the CP450 receptor. Already the NO receptor presented four significant correlations (between 0.48 and 0.74), while the MP receptor had two significant correlations (both 0.60). Relatively low correlation values may be considered non-significant according to the literature [ 21 , 22 ]. This shows that some of these descriptors have significant relation (values in bold) with the Ki values, (Table 2), reaffirming again the notable antioxidant ability. Total surface area (TSA) of a molecule is related to its solubility and the molecular interaction that it can performed due to the superficial contact with other molecules [ 23 ]. Similar analysis can be done for the molar volume (MV), because the higher the TSA and MV, the higher the level of interaction. From TSA and MV the intermolecular forces operate, giving rise to the energy of molecular interaction [ 24 ]. The Ki values showed significant and negative correlations with TSA and MV only in CP450 and NO, indicating that the increase in these descriptors values is important for the Ki to decrease (increased antioxidant ability) in the mentioned receptors. Molecules formed by different atoms have a polar covalent bond that can be quantified by a dipole moment. This is described with a positive partial charge close to a negative partial charge of the same absolute value, and the measure of the magnitude of these charges is given in Debyes (D). In the case of polyatomic molecules, a total dipole moment (TDM) is observed. The higher the TDM, the greater Molecules 2018,23, 2801 8 of 17 the polarity of the molecule and consequently greater a tendency to dissolve and interact in polar environment [ 25 , 26 ]. The electronegativity ( χ ), is directly related to TDM, and is described as the capacity that an atom has to attract to itself electrons of a bond that it makes with another atom [ 27 ]. It can be used to estimate the tendency of a molecule to attract electrons from another molecule with which it performs interaction [28]. Table 2. Molecular descriptors of the tested (Z20 and Z91) and reference molecules (5-fluorouracil—FLU, melatonin—MEL and dextromethorphan) and their correlations (CKi-CP450, CKi-MP and CKi-NO) with Ki values resulting from the molecular docking study. Descriptors Z91 Z20 FLU MEL DEX AST (Å2)457.6300 432.0800 229.5500 438.0500 371.1200 MV (Å3)831.4000 817.9600 348.7100 740.5100 821.6600 χ(eV) 2.9553 2.9776 3.5946 1.9003 1.9594 η(eV) 5.2982 5.3058 6.3539 5.4811 5.9292 1/η(eV) 0.1887 0.1885 0.1574 0.1824 0.1687 µ(eV) −2.9553 −2.9776 −3.5946 −1.9003 −1.9594 Ki-CP450 (µM) 24.30 17.89 182.0 - - Ki-MP (µM) 22.58 19.97 - 185.0 - Ki-NO (µM) 9.83 3.36 - - 7.11 Descriptors CKi-CP450 CKi-MP CKi-NO - - AST −0.95 −0.28 −0.51 - - VM −0.99 −0.05 −0.74 - - χ−0.67 −0.60 −0.08 - - η0.85 0.19 0.49 - - 1/η−0.83 −0.17 −0.48 - - µ−0.67 −0.60 −0.08 - - Å = Angstrom; eV = eletron volt. Significant data at p< 0.05. The Ki values showed significative negative (inversely proportional) correlations with χ only the CP450 and MP receptors. The χ values increase with the Ki values to decrease. An increase in the χ promotes an increase in the polarity with consequent increase in attractive forces (between receptor and other amino acid residues) in certain regions providing better receptor-ligand interaction—Ki (good binding affinity for antioxidant ability). The molecular hardness ( η ) and softness (1/ η ) also are important parameters that describe the reactivity of a molecule. The softness is related to basicity and to electron donation with high polarizability and low electronegativity, besides favoring the molecular flexibility with consequent chemical reactivity [ 29 – 31 ]. The hardness is already characterized by high ionization potential and high electronegativity, favoring the molecular stiffness with consequent chemical stability [32,33]. The descriptors η and 1/ η presented significant values of Ki knockouts only at CP450 and NO receptors. In these receptors the 1/ η values presented negative correlations with Ki, while the correlations between η and Ki were positive. Decrease of Ki with increase of the softness and decrease of the hardness, indicates the reactivity of the tested molecules and their contribution to the antioxidant ability (significant correlations with Ki). Chemical potential ( µ ) of a species is expressed as a function of thermodynamic quantities. And it can be described as a form of energy absorbed or released from a chemical reaction or change of state. It is related to free energy, binding affinity and inhibition constant, being influenced by the number of atoms or molecules that are added or subtracted from the system [ 34 – 36 ]. Negative values of correlation between µ and Ki were significant only for CP450 and MP. These negative values show that the higher the values of µ , the lower the Ki values (higher values of µ favor the antioxidant ability). The tested molecules presented values of µ close to those of the control molecules, favoring the antioxidant ability in the two mentioned receptors. Molecules 2018,23, 2801 9 of 17 2.3. Molecular Orbitals and Characteristic of Antioxidant Ability Table 3shows data and representations of the molecular orbitals, GAP values (energy variation between frontier orbitals) between all the energy states GAP 1 , GAP 2 , GAP 3 and GAP 4 . The molecular orbitals LUMO and LUMO +1 have a direct relation with the electron affinity of a molecule, which is related to susceptibility to nucleophilic attacks [ 37 ]. The values of LUMO and LUMO+1 for Z91 and Z20 were close to those of the control molecules. This show that Z91 and Z20 are less susceptible to attack by nucleophiles. The values of LUMO and LUMO+1 were positively correlated with the Ki values. Only CP450 receptors (0.91 for LUMO and 0.88 for LUMO+1) and MP (0.54 for LUMO and 0.60 for LUMO+1) showed significant correlation values, revealing the tendency of the Ki values to decrease with the reduction of LUMO and LUMO+1 values for these receptors. Table 3. Representations of the molecular orbitals and their GAP values (energy variation between frontier orbitals—HOMO and LUMO) between all energy states ([a] GAP1,[b] GAP2, [c] GAP3 and [d] GAP4) of the Z20, Z91, 5-fluorouracil (FLU), melatonin (MEL) and dextromethorphan (DEX). # LUMO+1 LUMO HOMO HOMO-1 Z20 Molecules 2018, 23, x 9 of 17 2.3. Molecular Orbitals and Characteristic of Antioxidant Ability Table 3 shows data and representations of the molecular orbitals, GAP values (energy variation between frontier orbitals) between all the energy states GAP 1 , GAP 2 , GAP 3 and GAP 4 . The molecular orbitals LUMO and LUMO +1 have a direct relation with the electron affinity of a molecule, which is related to susceptibility to nucleophilic attacks [37]. The values of LUMO and LUMO+1 for Z91 and Z20 were close to those of the control molecules. This show that Z91 and Z20 are less susceptible to attack by nucleophiles. The values of LUMO and LUMO+1 were positively correlated with the Ki values. Only CP450 receptors (0.91 for LUMO and 0.88 for LUMO+1) and MP (0.54 for LUMO and 0.60 for LUMO+1) showed significant correlation values, revealing the tendency of the Ki values to decrease with the reduction of LUMO and LUMO+1 values for these receptors. Table 3. Representations of the molecular orbitals and their GAP values (energy variation between frontier orbitals—HOMO and LUMO) between all energy states ( [a] GAP 1 , [b] GAP2, [c] GAP3 and [d] GAP4) of the Z20, Z91, 5-fluorouracil (FLU), melatonin (MEL) and dextromethorphan (DEX). [a] GAP1 = LUMO − HOMO; [b] GAP2 = LUMO+1 – HOMO; [c] GAP3 =LUMO – HOMO-1; [d] GAP4 = LUMO+1 – HOMO-1. Significant correlation values with Ki were obtained for molecular orbital data (see Table 4). The HOMO orbital showed a significant correlation with Ki only in the MP receptor (0.47) and the # LUMO+1 LUMO HOMO HOMO-1 Z20 4.3201 eV 2.3282 eV −8.2834 eV −9.4429 eV GAP1 = 10.6116 eV GAP2 = 12.6035 eV GAP3 = 11.7711 eV GAP4 = 13.7630 eV Z91 4.3397 eV 2.3429 eV −8.2535 eV −9.4143 eV GAP1 = 10.5964 eV GAP2 = 12.5932 eV GAP3 = 11.7572 eV GAP4 = 13.7540 eV FLU 4.9438 eV 2.7592 eV −9.9485 eV −12.0353 eV GAP1 = 12.7077 eV GAP2 = 14.8923 eV GAP3 = 14.7945 eV GAP4 = 16.9791 eV MEL 5.2681 eV 3.5807 eV −7.3814 eV −7.6992 eV GAP1 = 10.9621 eV GAP2 = 12.6495 eV GAP3 = 11.2799 eV GAP4 = 12.9853 eV DEX 4.5800 eV 3.9699 eV −7.8886 eV −8.7485 eV GAP1 = 11.8585 eV GAP2 = 12.4686 eV GAP3 = 12.4479 eV GAP4 = 13.3285 eV Molecules 2018, 23, x 9 of 17 2.3. Molecular Orbitals and Characteristic of Antioxidant Ability Table 3 shows data and representations of the molecular orbitals, GAP values (energy variation between frontier orbitals) between all the energy states GAP 1 , GAP 2 , GAP 3 and GAP 4 . The molecular orbitals LUMO and LUMO +1 have a direct relation with the electron affinity of a molecule, which is related to susceptibility to nucleophilic attacks [37]. The values of LUMO and LUMO+1 for Z91 and Z20 were close to those of the control molecules. This show that Z91 and Z20 are less susceptible to attack by nucleophiles. The values of LUMO and LUMO+1 were positively correlated with the Ki values. Only CP450 receptors (0.91 for LUMO and 0.88 for LUMO+1) and MP (0.54 for LUMO and 0.60 for LUMO+1) showed significant correlation values, revealing the tendency of the Ki values to decrease with the reduction of LUMO and LUMO+1 values for these receptors. Table 3. Representations of the molecular orbitals and their GAP values (energy variation between frontier orbitals—HOMO and LUMO) between all energy states ( [a] GAP 1 , [b] GAP2, [c] GAP3 and [d] GAP4) of the Z20, Z91, 5-fluorouracil (FLU), melatonin (MEL) and dextromethorphan (DEX). [a] GAP1 = LUMO − HOMO; [b] GAP2 = LUMO+1 – HOMO; [c] GAP3 =LUMO – HOMO-1; [d] GAP4 = LUMO+1 – HOMO-1. Significant correlation values with Ki were obtained for molecular orbital data (see Table 4). The HOMO orbital showed a significant correlation with Ki only in the MP receptor (0.47) and the # LUMO+1 LUMO HOMO HOMO-1 Z20 4.3201 eV 2.3282 eV −8.2834 eV −9.4429 eV GAP1 = 10.6116 eV GAP2 = 12.6035 eV GAP3 = 11.7711 eV GAP4 = 13.7630 eV Z91 4.3397 eV 2.3429 eV −8.2535 eV −9.4143 eV GAP1 = 10.5964 eV GAP2 = 12.5932 eV GAP3 = 11.7572 eV GAP4 = 13.7540 eV FLU 4.9438 eV 2.7592 eV −9.9485 eV −12.0353 eV GAP1 = 12.7077 eV GAP2 = 14.8923 eV GAP3 = 14.7945 eV GAP4 = 16.9791 eV MEL 5.2681 eV 3.5807 eV −7.3814 eV −7.6992 eV GAP1 = 10.9621 eV GAP2 = 12.6495 eV GAP3 = 11.2799 eV GAP4 = 12.9853 eV DEX 4.5800 eV 3.9699 eV −7.8886 eV −8.7485 eV GAP1 = 11.8585 eV GAP2 = 12.4686 eV GAP3 = 12.4479 eV GAP4 = 13.3285 eV Molecules 2018, 23, x 9 of 17 2.3. Molecular Orbitals and Characteristic of Antioxidant Ability Table 3 shows data and representations of the molecular orbitals, GAP values (energy variation between frontier orbitals) between all the energy states GAP 1 , GAP 2 , GAP 3 and GAP 4 . The molecular orbitals LUMO and LUMO +1 have a direct relation with the electron affinity of a molecule, which is related to susceptibility to nucleophilic attacks [37]. The values of LUMO and LUMO+1 for Z91 and Z20 were close to those of the control molecules. This show that Z91 and Z20 are less susceptible to attack by nucleophiles. The values of LUMO and LUMO+1 were positively correlated with the Ki values. Only CP450 receptors (0.91 for LUMO and 0.88 for LUMO+1) and MP (0.54 for LUMO and 0.60 for LUMO+1) showed significant correlation values, revealing the tendency of the Ki values to decrease with the reduction of LUMO and LUMO+1 values for these receptors. Table 3. Representations of the molecular orbitals and their GAP values (energy variation between frontier orbitals—HOMO and LUMO) between all energy states ( [a] GAP 1 , [b] GAP2, [c] GAP3 and [d] GAP4) of the Z20, Z91, 5-fluorouracil (FLU), melatonin (MEL) and dextromethorphan (DEX). [a] GAP1 = LUMO − HOMO; [b] GAP2 = LUMO+1 – HOMO; [c] GAP3 =LUMO – HOMO-1; [d] GAP4 = LUMO+1 – HOMO-1. Significant correlation values with Ki were obtained for molecular orbital data (see Table 4). The HOMO orbital showed a significant correlation with Ki only in the MP receptor (0.47) and the # LUMO+1 LUMO HOMO HOMO-1 Z20 4.3201 eV 2.3282 eV −8.2834 eV −9.4429 eV GAP1 = 10.6116 eV GAP2 = 12.6035 eV GAP3 = 11.7711 eV GAP4 = 13.7630 eV Z91 4.3397 eV 2.3429 eV −8.2535 eV −9.4143 eV GAP1 = 10.5964 eV GAP2 = 12.5932 eV GAP3 = 11.7572 eV GAP4 = 13.7540 eV FLU 4.9438 eV 2.7592 eV −9.9485 eV −12.0353 eV GAP1 = 12.7077 eV GAP2 = 14.8923 eV GAP3 = 14.7945 eV GAP4 = 16.9791 eV MEL 5.2681 eV 3.5807 eV −7.3814 eV −7.6992 eV GAP1 = 10.9621 eV GAP2 = 12.6495 eV GAP3 = 11.2799 eV GAP4 = 12.9853 eV DEX 4.5800 eV 3.9699 eV −7.8886 eV −8.7485 eV GAP1 = 11.8585 eV GAP2 = 12.4686 eV GAP3 = 12.4479 eV GAP4 = 13.3285 eV Molecules 2018, 23, x 9 of 17 2.3. Molecular Orbitals and Characteristic of Antioxidant Ability Table 3 shows data and representations of the molecular orbitals, GAP values (energy variation between frontier orbitals) between all the energy states GAP 1 , GAP 2 , GAP 3 and GAP 4 . The molecular orbitals LUMO and LUMO +1 have a direct relation with the electron affinity of a molecule, which is related to susceptibility to nucleophilic attacks [37]. The values of LUMO and LUMO+1 for Z91 and Z20 were close to those of the control molecules. This show that Z91 and Z20 are less susceptible to attack by nucleophiles. The values of LUMO and LUMO+1 were positively correlated with the Ki values. Only CP450 receptors (0.91 for LUMO and 0.88 for LUMO+1) and MP (0.54 for LUMO and 0.60 for LUMO+1) showed significant correlation values, revealing the tendency of the Ki values to decrease with the reduction of LUMO and LUMO+1 values for these receptors. Table 3. Representations of the molecular orbitals and their GAP values (energy variation between frontier orbitals—HOMO and LUMO) between all energy states ( [a] GAP 1 , [b] GAP2, [c] GAP3 and [d] GAP4) of the Z20, Z91, 5-fluorouracil (FLU), melatonin (MEL) and dextromethorphan (DEX). [a] GAP1 = LUMO − HOMO; [b] GAP2 = LUMO+1 – HOMO; [c] GAP3 =LUMO – HOMO-1; [d] GAP4 = LUMO+1 – HOMO-1. Significant correlation values with Ki were obtained for molecular orbital data (see Table 4). The HOMO orbital showed a significant correlation with Ki only in the MP receptor (0.47) and the # LUMO+1 LUMO HOMO HOMO-1 Z20 4.3201 eV 2.3282 eV −8.2834 eV −9.4429 eV GAP1 = 10.6116 eV GAP2 = 12.6035 eV GAP3 = 11.7711 eV GAP4 = 13.7630 eV Z91 4.3397 eV 2.3429 eV −8.2535 eV −9.4143 eV GAP1 = 10.5964 eV GAP2 = 12.5932 eV GAP3 = 11.7572 eV GAP4 = 13.7540 eV FLU 4.9438 eV 2.7592 eV −9.9485 eV −12.0353 eV GAP1 = 12.7077 eV GAP2 = 14.8923 eV GAP3 = 14.7945 eV GAP4 = 16.9791 eV MEL 5.2681 eV 3.5807 eV −7.3814 eV −7.6992 eV GAP1 = 10.9621 eV GAP2 = 12.6495 eV GAP3 = 11.2799 eV GAP4 = 12.9853 eV DEX 4.5800 eV 3.9699 eV −7.8886 eV −8.7485 eV GAP1 = 11.8585 eV GAP2 = 12.4686 eV GAP3 = 12.4479 eV GAP4 = 13.3285 eV GAP1 = 10.6116 eV GAP2 = 12.6035 eV GAP3 = 11.7711 eV GAP4 = 13.7630 eV Z91 Molecules 2018, 23, x 9 of 17 2.3. Molecular Orbitals and Characteristic of Antioxidant Ability Table 3 shows data and representations of the molecular orbitals, GAP values (energy variation between frontier orbitals) between all the energy states GAP 1 , GAP 2 , GAP 3 and GAP 4 . The molecular orbitals LUMO and LUMO +1 have a direct relation with the electron affinity of a molecule, which is related to susceptibility to nucleophilic attacks [37]. The values of LUMO and LUMO+1 for Z91 and Z20 were close to those of the control molecules. This show that Z91 and Z20 are less susceptible to attack by nucleophiles. The values of LUMO and LUMO+1 were positively correlated with the Ki values. Only CP450 receptors (0.91 for LUMO and 0.88 for LUMO+1) and MP (0.54 for LUMO and 0.60 for LUMO+1) showed significant correlation values, revealing the tendency of the Ki values to decrease with the reduction of LUMO and LUMO+1 values for these receptors. Table 3. Representations of the molecular orbitals and their GAP values (energy variation between frontier orbitals—HOMO and LUMO) between all energy states ( [a] GAP 1 , [b] GAP2, [c] GAP3 and [d] GAP4) of the Z20, Z91, 5-fluorouracil (FLU), melatonin (MEL) and dextromethorphan (DEX). [a] GAP1 = LUMO − HOMO; [b] GAP2 = LUMO+1 – HOMO; [c] GAP3 =LUMO – HOMO-1; [d] GAP4 = LUMO+1 – HOMO-1. Significant correlation values with Ki were obtained for molecular orbital data (see Table 4). The HOMO orbital showed a significant correlation with Ki only in the MP receptor (0.47) and the # LUMO+1 LUMO HOMO HOMO-1 Z20 4.3201 eV 2.3282 eV −8.2834 eV −9.4429 eV GAP1 = 10.6116 eV GAP2 = 12.6035 eV GAP3 = 11.7711 eV GAP4 = 13.7630 eV Z91 4.3397 eV 2.3429 eV −8.2535 eV −9.4143 eV GAP1 = 10.5964 eV GAP2 = 12.5932 eV GAP3 = 11.7572 eV GAP4 = 13.7540 eV FLU 4.9438 eV 2.7592 eV −9.9485 eV −12.0353 eV GAP1 = 12.7077 eV GAP2 = 14.8923 eV GAP3 = 14.7945 eV GAP4 = 16.9791 eV MEL 5.2681 eV 3.5807 eV −7.3814 eV −7.6992 eV GAP1 = 10.9621 eV GAP2 = 12.6495 eV GAP3 = 11.2799 eV GAP4 = 12.9853 eV DEX 4.5800 eV 3.9699 eV −7.8886 eV −8.7485 eV GAP1 = 11.8585 eV GAP2 = 12.4686 eV GAP3 = 12.4479 eV GAP4 = 13.3285 eV Molecules 2018, 23, x 9 of 17 2.3. Molecular Orbitals and Characteristic of Antioxidant Ability Table 3 shows data and representations of the molecular orbitals, GAP values (energy variation between frontier orbitals) between all the energy states GAP 1 , GAP 2 , GAP 3 and GAP 4 . The molecular orbitals LUMO and LUMO +1 have a direct relation with the electron affinity of a molecule, which is related to susceptibility to nucleophilic attacks [37]. The values of LUMO and LUMO+1 for Z91 and Z20 were close to those of the control molecules. This show that Z91 and Z20 are less susceptible to attack by nucleophiles. The values of LUMO and LUMO+1 were positively correlated with the Ki values. Only CP450 receptors (0.91 for LUMO and 0.88 for LUMO+1) and MP (0.54 for LUMO and 0.60 for LUMO+1) showed significant correlation values, revealing the tendency of the Ki values to decrease with the reduction of LUMO and LUMO+1 values for these receptors. Table 3. Representations of the molecular orbitals and their GAP values (energy variation between frontier orbitals—HOMO and LUMO) between all energy states ( [a] GAP 1 , [b] GAP2, [c] GAP3 and [d] GAP4) of the Z20, Z91, 5-fluorouracil (FLU), melatonin (MEL) and dextromethorphan (DEX). [a] GAP1 = LUMO − HOMO; [b] GAP2 = LUMO+1 – HOMO; [c] GAP3 =LUMO – HOMO-1; [d] GAP4 = LUMO+1 – HOMO-1. Significant correlation values with Ki were obtained for molecular orbital data (see Table 4). The HOMO orbital showed a significant correlation with Ki only in the MP receptor (0.47) and the # LUMO+1 LUMO HOMO HOMO-1 Z20 4.3201 eV 2.3282 eV −8.2834 eV −9.4429 eV GAP1 = 10.6116 eV GAP2 = 12.6035 eV GAP3 = 11.7711 eV GAP4 = 13.7630 eV Z91 4.3397 eV 2.3429 eV −8.2535 eV −9.4143 eV GAP1 = 10.5964 eV GAP2 = 12.5932 eV GAP3 = 11.7572 eV GAP4 = 13.7540 eV FLU 4.9438 eV 2.7592 eV −9.9485 eV −12.0353 eV GAP1 = 12.7077 eV GAP2 = 14.8923 eV GAP3 = 14.7945 eV GAP4 = 16.9791 eV MEL 5.2681 eV 3.5807 eV −7.3814 eV −7.6992 eV GAP1 = 10.9621 eV GAP2 = 12.6495 eV GAP3 = 11.2799 eV GAP4 = 12.9853 eV DEX 4.5800 eV 3.9699 eV −7.8886 eV −8.7485 eV GAP1 = 11.8585 eV GAP2 = 12.4686 eV GAP3 = 12.4479 eV GAP4 = 13.3285 eV Molecules 2018, 23, x 9 of 18 2.3. Molecular Orbitals and Characteristic of Antioxidant Ability Table 3 shows data and representations of the molecular orbitals, GAP values (energy variation between frontier orbitals) between all the energy states GAP1, GAP2, GAP3 and GAP4. The molecular orbitals LUMO and LUMO +1 have a direct relation with the electron affinity of a molecule, which is related to susceptibility to nucleophilic attacks [37]. The values of LUMO and LUMO+1 for Z91 and Z20 were close to those of the control molecules. This show that Z91 and Z20 are less susceptible to attack by nucleophiles. The values of LUMO and LUMO+1 were positively correlated with the Ki values. Only CP450 receptors (0.91 for LUMO and 0.88 for LUMO+1) and MP (0.54 for LUMO and 0.60 for LUMO+1) showed significant correlation values, revealing the tendency of the Ki values to decrease with the reduction of LUMO and LUMO+1 values for these receptors. Table 3. Representations of the molecular orbitals and their GAP values (energy variation between frontier orbitals—HOMO and LUMO) between all energy states ([a] GAP1, [b] GAP2, [c] GAP3 and [d] GAP4) of the Z20, Z91, 5-fluorouracil (FLU), melatonin (MEL) and dextromethorphan (DEX). [a] GAP1 = LUMO − HOMO; [b] GAP2 = LUMO+1 – HOMO; [c] GAP3 =LUMO – HOMO-1; [d] GAP4 = LUMO+1 – HOMO-1. # LUMO+1 LUMO HOMO HOMO-1 Z20 4.3201 eV 2.3282 eV −8.2834 eV −9.4429 eV GAP1 = 10.6116 eV GAP2 = 12.6035 eV GAP3 = 11.7711 eV GAP4 = 13.7630 eV Z91 4.3397 eV 2.3429 eV −8.2535 eV −9.4143 eV GAP1 = 10.5964 eV GAP2 = 12.5932 eV GAP3 = 11.7572 eV GAP4 = 13.7540 eV FLU 4.9438 eV 2.7592 eV −9.9485 eV −12.0353 eV GAP1 = 12.7077 eV GAP2 = 14.8923 eV GAP3 = 14.7945 eV GAP4 = 16.9791 eV MEL 5.2681 eV 3.5807 eV −7.3814 eV −7.6992 eV GAP1 = 10.9621 eV GAP2 = 12.6495 eV GAP3 = 11.2799 eV GAP4 = 12.9853 eV DEX 4.5800 eV 3.9699 eV −7.8886 eV −8.7485 eV GAP1 = 11.8585 eV GAP2 = 12.4686 eV GAP3 = 12.4479 eV GAP4 = 13.3285 eV Molecules 2018, 23, x 9 of 17 2.3. Molecular Orbitals and Characteristic of Antioxidant Ability Table 3 shows data and representations of the molecular orbitals, GAP values (energy variation between frontier orbitals) between all the energy states GAP 1 , GAP 2 , GAP 3 and GAP 4 . The molecular orbitals LUMO and LUMO +1 have a direct relation with the electron affinity of a molecule, which is related to susceptibility to nucleophilic attacks [37]. The values of LUMO and LUMO+1 for Z91 and Z20 were close to those of the control molecules. This show that Z91 and Z20 are less susceptible to attack by nucleophiles. The values of LUMO and LUMO+1 were positively correlated with the Ki values. Only CP450 receptors (0.91 for LUMO and 0.88 for LUMO+1) and MP (0.54 for LUMO and 0.60 for LUMO+1) showed significant correlation values, revealing the tendency of the Ki values to decrease with the reduction of LUMO and LUMO+1 values for these receptors. Table 3. Representations of the molecular orbitals and their GAP values (energy variation between frontier orbitals—HOMO and LUMO) between all energy states ( [a] GAP 1 , [b] GAP2, [c] GAP3 and [d] GAP4) of the Z20, Z91, 5-fluorouracil (FLU), melatonin (MEL) and dextromethorphan (DEX). [a] GAP1 = LUMO − HOMO; [b] GAP2 = LUMO+1 – HOMO; [c] GAP3 =LUMO – HOMO-1; [d] GAP4 = LUMO+1 – HOMO-1. Significant correlation values with Ki were obtained for molecular orbital data (see Table 4). The HOMO orbital showed a significant correlation with Ki only in the MP receptor (0.47) and the # LUMO+1 LUMO HOMO HOMO-1 Z20 4.3201 eV 2.3282 eV −8.2834 eV −9.4429 eV GAP1 = 10.6116 eV GAP2 = 12.6035 eV GAP3 = 11.7711 eV GAP4 = 13.7630 eV Z91 4.3397 eV 2.3429 eV −8.2535 eV −9.4143 eV GAP1 = 10.5964 eV GAP2 = 12.5932 eV GAP3 = 11.7572 eV GAP4 = 13.7540 eV FLU 4.9438 eV 2.7592 eV −9.9485 eV −12.0353 eV GAP1 = 12.7077 eV GAP2 = 14.8923 eV GAP3 = 14.7945 eV GAP4 = 16.9791 eV MEL 5.2681 eV 3.5807 eV −7.3814 eV −7.6992 eV GAP1 = 10.9621 eV GAP2 = 12.6495 eV GAP3 = 11.2799 eV GAP4 = 12.9853 eV DEX 4.5800 eV 3.9699 eV −7.8886 eV −8.7485 eV GAP1 = 11.8585 eV GAP2 = 12.4686 eV GAP3 = 12.4479 eV GAP4 = 13.3285 eV GAP1 = 10.5964 eV GAP2 = 12.5932 eV GAP3 = 11.7572 eV GAP4 = 13.7540 eV FLU Molecules 2018, 23, x 9 of 17 2.3. Molecular Orbitals and Characteristic of Antioxidant Ability Table 3 shows data and representations of the molecular orbitals, GAP values (energy variation between frontier orbitals) between all the energy states GAP 1 , GAP 2 , GAP 3 and GAP 4 . The molecular orbitals LUMO and LUMO +1 have a direct relation with the electron affinity of a molecule, which is related to susceptibility to nucleophilic attacks [37]. The values of LUMO and LUMO+1 for Z91 and Z20 were close to those of the control molecules. This show that Z91 and Z20 are less susceptible to attack by nucleophiles. The values of LUMO and LUMO+1 were positively correlated with the Ki values. Only CP450 receptors (0.91 for LUMO and 0.88 for LUMO+1) and MP (0.54 for LUMO and 0.60 for LUMO+1) showed significant correlation values, revealing the tendency of the Ki values to decrease with the reduction of LUMO and LUMO+1 values for these receptors. Table 3. Representations of the molecular orbitals and their GAP values (energy variation between frontier orbitals—HOMO and LUMO) between all energy states ( [a] GAP 1 , [b] GAP2, [c] GAP3 and [d] GAP4) of the Z20, Z91, 5-fluorouracil (FLU), melatonin (MEL) and dextromethorphan (DEX). [a] GAP1 = LUMO − HOMO; [b] GAP2 = LUMO+1 – HOMO; [c] GAP3 =LUMO – HOMO-1; [d] GAP4 = LUMO+1 – HOMO-1. Significant correlation values with Ki were obtained for molecular orbital data (see Table 4). The HOMO orbital showed a significant correlation with Ki only in the MP receptor (0.47) and the # LUMO+1 LUMO HOMO HOMO-1 Z20 4.3201 eV 2.3282 eV −8.2834 eV −9.4429 eV GAP1 = 10.6116 eV GAP2 = 12.6035 eV GAP3 = 11.7711 eV GAP4 = 13.7630 eV Z91 4.3397 eV 2.3429 eV −8.2535 eV −9.4143 eV GAP1 = 10.5964 eV GAP2 = 12.5932 eV GAP3 = 11.7572 eV GAP4 = 13.7540 eV FLU 4.9438 eV 2.7592 eV −9.9485 eV −12.0353 eV GAP1 = 12.7077 eV GAP2 = 14.8923 eV GAP3 = 14.7945 eV GAP4 = 16.9791 eV MEL 5.2681 eV 3.5807 eV −7.3814 eV −7.6992 eV GAP1 = 10.9621 eV GAP2 = 12.6495 eV GAP3 = 11.2799 eV GAP4 = 12.9853 eV DEX 4.5800 eV 3.9699 eV −7.8886 eV −8.7485 eV GAP1 = 11.8585 eV GAP2 = 12.4686 eV GAP3 = 12.4479 eV GAP4 = 13.3285 eV Molecules 2018, 23, x 9 of 17 2.3. Molecular Orbitals and Characteristic of Antioxidant Ability Table 3 shows data and representations of the molecular orbitals, GAP values (energy variation between frontier orbitals) between all the energy states GAP 1 , GAP 2 , GAP 3 and GAP 4 . The molecular orbitals LUMO and LUMO +1 have a direct relation with the electron affinity of a molecule, which is related to susceptibility to nucleophilic attacks [37]. The values of LUMO and LUMO+1 for Z91 and Z20 were close to those of the control molecules. This show that Z91 and Z20 are less susceptible to attack by nucleophiles. The values of LUMO and LUMO+1 were positively correlated with the Ki values. Only CP450 receptors (0.91 for LUMO and 0.88 for LUMO+1) and MP (0.54 for LUMO and 0.60 for LUMO+1) showed significant correlation values, revealing the tendency of the Ki values to decrease with the reduction of LUMO and LUMO+1 values for these receptors. Table 3. Representations of the molecular orbitals and their GAP values (energy variation between frontier orbitals—HOMO and LUMO) between all energy states ( [a] GAP 1 , [b] GAP2, [c] GAP3 and [d] GAP4) of the Z20, Z91, 5-fluorouracil (FLU), melatonin (MEL) and dextromethorphan (DEX). [a] GAP1 = LUMO − HOMO; [b] GAP2 = LUMO+1 – HOMO; [c] GAP3 =LUMO – HOMO-1; [d] GAP4 = LUMO+1 – HOMO-1. Significant correlation values with Ki were obtained for molecular orbital data (see Table 4). The HOMO orbital showed a significant correlation with Ki only in the MP receptor (0.47) and the # LUMO+1 LUMO HOMO HOMO-1 Z20 4.3201 eV 2.3282 eV −8.2834 eV −9.4429 eV GAP1 = 10.6116 eV GAP2 = 12.6035 eV GAP3 = 11.7711 eV GAP4 = 13.7630 eV Z91 4.3397 eV 2.3429 eV −8.2535 eV −9.4143 eV GAP1 = 10.5964 eV GAP2 = 12.5932 eV GAP3 = 11.7572 eV GAP4 = 13.7540 eV FLU 4.9438 eV 2.7592 eV −9.9485 eV −12.0353 eV GAP1 = 12.7077 eV GAP2 = 14.8923 eV GAP3 = 14.7945 eV GAP4 = 16.9791 eV MEL 5.2681 eV 3.5807 eV −7.3814 eV −7.6992 eV GAP1 = 10.9621 eV GAP2 = 12.6495 eV GAP3 = 11.2799 eV GAP4 = 12.9853 eV DEX 4.5800 eV 3.9699 eV −7.8886 eV −8.7485 eV GAP1 = 11.8585 eV GAP2 = 12.4686 eV GAP3 = 12.4479 eV GAP4 = 13.3285 eV Molecules 2018, 23, x 9 of 17 2.3. Molecular Orbitals and Characteristic of Antioxidant Ability Table 3 shows data and representations of the molecular orbitals, GAP values (energy variation between frontier orbitals) between all the energy states GAP 1 , GAP 2 , GAP 3 and GAP 4 . The molecular orbitals LUMO and LUMO +1 have a direct relation with the electron affinity of a molecule, which is related to susceptibility to nucleophilic attacks [37]. The values of LUMO and LUMO+1 for Z91 and Z20 were close to those of the control molecules. This show that Z91 and Z20 are less susceptible to attack by nucleophiles. The values of LUMO and LUMO+1 were positively correlated with the Ki values. Only CP450 receptors (0.91 for LUMO and 0.88 for LUMO+1) and MP (0.54 for LUMO and 0.60 for LUMO+1) showed significant correlation values, revealing the tendency of the Ki values to decrease with the reduction of LUMO and LUMO+1 values for these receptors. Table 3. Representations of the molecular orbitals and their GAP values (energy variation between frontier orbitals—HOMO and LUMO) between all energy states ( [a] GAP 1 , [b] GAP2, [c] GAP3 and [d] GAP4) of the Z20, Z91, 5-fluorouracil (FLU), melatonin (MEL) and dextromethorphan (DEX). [a] GAP1 = LUMO − HOMO; [b] GAP2 = LUMO+1 – HOMO; [c] GAP3 =LUMO – HOMO-1; [d] GAP4 = LUMO+1 – HOMO-1. Significant correlation values with Ki were obtained for molecular orbital data (see Table 4). The HOMO orbital showed a significant correlation with Ki only in the MP receptor (0.47) and the # LUMO+1 LUMO HOMO HOMO-1 Z20 4.3201 eV 2.3282 eV −8.2834 eV −9.4429 eV GAP1 = 10.6116 eV GAP2 = 12.6035 eV GAP3 = 11.7711 eV GAP4 = 13.7630 eV Z91 4.3397 eV 2.3429 eV −8.2535 eV −9.4143 eV GAP1 = 10.5964 eV GAP2 = 12.5932 eV GAP3 = 11.7572 eV GAP4 = 13.7540 eV FLU 4.9438 eV 2.7592 eV −9.9485 eV −12.0353 eV GAP1 = 12.7077 eV GAP2 = 14.8923 eV GAP3 = 14.7945 eV GAP4 = 16.9791 eV MEL 5.2681 eV 3.5807 eV −7.3814 eV −7.6992 eV GAP1 = 10.9621 eV GAP2 = 12.6495 eV GAP3 = 11.2799 eV GAP4 = 12.9853 eV DEX 4.5800 eV 3.9699 eV −7.8886 eV −8.7485 eV GAP1 = 11.8585 eV GAP2 = 12.4686 eV GAP3 = 12.4479 eV GAP4 = 13.3285 eV Molecules 2018, 23, x 9 of 17 2.3. Molecular Orbitals and Characteristic of Antioxidant Ability Table 3 shows data and representations of the molecular orbitals, GAP values (energy variation between frontier orbitals) between all the energy states GAP 1 , GAP 2 , GAP 3 and GAP 4 . The molecular orbitals LUMO and LUMO +1 have a direct relation with the electron affinity of a molecule, which is related to susceptibility to nucleophilic attacks [37]. The values of LUMO and LUMO+1 for Z91 and Z20 were close to those of the control molecules. This show that Z91 and Z20 are less susceptible to attack by nucleophiles. The values of LUMO and LUMO+1 were positively correlated with the Ki values. Only CP450 receptors (0.91 for LUMO and 0.88 for LUMO+1) and MP (0.54 for LUMO and 0.60 for LUMO+1) showed significant correlation values, revealing the tendency of the Ki values to decrease with the reduction of LUMO and LUMO+1 values for these receptors. Table 3. Representations of the molecular orbitals and their GAP values (energy variation between frontier orbitals—HOMO and LUMO) between all energy states ( [a] GAP 1 , [b] GAP2, [c] GAP3 and [d] GAP4) of the Z20, Z91, 5-fluorouracil (FLU), melatonin (MEL) and dextromethorphan (DEX). [a] GAP1 = LUMO − HOMO; [b] GAP2 = LUMO+1 – HOMO; [c] GAP3 =LUMO – HOMO-1; [d] GAP4 = LUMO+1 – HOMO-1. Significant correlation values with Ki were obtained for molecular orbital data (see Table 4). The HOMO orbital showed a significant correlation with Ki only in the MP receptor (0.47) and the # LUMO+1 LUMO HOMO HOMO-1 Z20 4.3201 eV 2.3282 eV −8.2834 eV −9.4429 eV GAP1 = 10.6116 eV GAP2 = 12.6035 eV GAP3 = 11.7711 eV GAP4 = 13.7630 eV Z91 4.3397 eV 2.3429 eV −8.2535 eV −9.4143 eV GAP1 = 10.5964 eV GAP2 = 12.5932 eV GAP3 = 11.7572 eV GAP4 = 13.7540 eV FLU 4.9438 eV 2.7592 eV −9.9485 eV −12.0353 eV GAP1 = 12.7077 eV GAP2 = 14.8923 eV GAP3 = 14.7945 eV GAP4 = 16.9791 eV MEL 5.2681 eV 3.5807 eV −7.3814 eV −7.6992 eV GAP1 = 10.9621 eV GAP2 = 12.6495 eV GAP3 = 11.2799 eV GAP4 = 12.9853 eV DEX 4.5800 eV 3.9699 eV −7.8886 eV −8.7485 eV GAP1 = 11.8585 eV GAP2 = 12.4686 eV GAP3 = 12.4479 eV GAP4 = 13.3285 eV GAP1 = 12.7077 eV GAP2 = 14.8923 eV GAP3 = 14.7945 eV GAP4 = 16.9791 eV MEL Molecules 2018, 23, x 9 of 17 2.3. Molecular Orbitals and Characteristic of Antioxidant Ability Table 3 shows data and representations of the molecular orbitals, GAP values (energy variation between frontier orbitals) between all the energy states GAP 1 , GAP 2 , GAP 3 and GAP 4 . The molecular orbitals LUMO and LUMO +1 have a direct relation with the electron affinity of a molecule, which is related to susceptibility to nucleophilic attacks [37]. The values of LUMO and LUMO+1 for Z91 and Z20 were close to those of the control molecules. This show that Z91 and Z20 are less susceptible to attack by nucleophiles. The values of LUMO and LUMO+1 were positively correlated with the Ki values. Only CP450 receptors (0.91 for LUMO and 0.88 for LUMO+1) and MP (0.54 for LUMO and 0.60 for LUMO+1) showed significant correlation values, revealing the tendency of the Ki values to decrease with the reduction of LUMO and LUMO+1 values for these receptors. Table 3. Representations of the molecular orbitals and their GAP values (energy variation between frontier orbitals—HOMO and LUMO) between all energy states ( [a] GAP 1 , [b] GAP2, [c] GAP3 and [d] GAP4) of the Z20, Z91, 5-fluorouracil (FLU), melatonin (MEL) and dextromethorphan (DEX). [a] GAP1 = LUMO − HOMO; [b] GAP2 = LUMO+1 – HOMO; [c] GAP3 =LUMO – HOMO-1; [d] GAP4 = LUMO+1 – HOMO-1. Significant correlation values with Ki were obtained for molecular orbital data (see Table 4). The HOMO orbital showed a significant correlation with Ki only in the MP receptor (0.47) and the # LUMO+1 LUMO HOMO HOMO-1 Z20 4.3201 eV 2.3282 eV −8.2834 eV −9.4429 eV GAP1 = 10.6116 eV GAP2 = 12.6035 eV GAP3 = 11.7711 eV GAP4 = 13.7630 eV Z91 4.3397 eV 2.3429 eV −8.2535 eV −9.4143 eV GAP1 = 10.5964 eV GAP2 = 12.5932 eV GAP3 = 11.7572 eV GAP4 = 13.7540 eV FLU 4.9438 eV 2.7592 eV −9.9485 eV −12.0353 eV GAP1 = 12.7077 eV GAP2 = 14.8923 eV GAP3 = 14.7945 eV GAP4 = 16.9791 eV MEL 5.2681 eV 3.5807 eV −7.3814 eV −7.6992 eV GAP1 = 10.9621 eV GAP2 = 12.6495 eV GAP3 = 11.2799 eV GAP4 = 12.9853 eV DEX 4.5800 eV 3.9699 eV −7.8886 eV −8.7485 eV GAP1 = 11.8585 eV GAP2 = 12.4686 eV GAP3 = 12.4479 eV GAP4 = 13.3285 eV Molecules 2018, 23, x 9 of 17 2.3. Molecular Orbitals and Characteristic of Antioxidant Ability Table 3 shows data and representations of the molecular orbitals, GAP values (energy variation between frontier orbitals) between all the energy states GAP 1 , GAP 2 , GAP 3 and GAP 4 . The molecular orbitals LUMO and LUMO +1 have a direct relation with the electron affinity of a molecule, which is related to susceptibility to nucleophilic attacks [37]. The values of LUMO and LUMO+1 for Z91 and Z20 were close to those of the control molecules. This show that Z91 and Z20 are less susceptible to attack by nucleophiles. The values of LUMO and LUMO+1 were positively correlated with the Ki values. Only CP450 receptors (0.91 for LUMO and 0.88 for LUMO+1) and MP (0.54 for LUMO and 0.60 for LUMO+1) showed significant correlation values, revealing the tendency of the Ki values to decrease with the reduction of LUMO and LUMO+1 values for these receptors. Table 3. Representations of the molecular orbitals and their GAP values (energy variation between frontier orbitals—HOMO and LUMO) between all energy states ( [a] GAP 1 , [b] GAP2, [c] GAP3 and [d] GAP4) of the Z20, Z91, 5-fluorouracil (FLU), melatonin (MEL) and dextromethorphan (DEX). [a] GAP1 = LUMO − HOMO; [b] GAP2 = LUMO+1 – HOMO; [c] GAP3 =LUMO – HOMO-1; [d] GAP4 = LUMO+1 – HOMO-1. Significant correlation values with Ki were obtained for molecular orbital data (see Table 4). The HOMO orbital showed a significant correlation with Ki only in the MP receptor (0.47) and the # LUMO+1 LUMO HOMO HOMO-1 Z20 4.3201 eV 2.3282 eV −8.2834 eV −9.4429 eV GAP1 = 10.6116 eV GAP2 = 12.6035 eV GAP3 = 11.7711 eV GAP4 = 13.7630 eV Z91 4.3397 eV 2.3429 eV −8.2535 eV −9.4143 eV GAP1 = 10.5964 eV GAP2 = 12.5932 eV GAP3 = 11.7572 eV GAP4 = 13.7540 eV FLU 4.9438 eV 2.7592 eV −9.9485 eV −12.0353 eV GAP1 = 12.7077 eV GAP2 = 14.8923 eV GAP3 = 14.7945 eV GAP4 = 16.9791 eV MEL 5.2681 eV 3.5807 eV −7.3814 eV −7.6992 eV GAP1 = 10.9621 eV GAP2 = 12.6495 eV GAP3 = 11.2799 eV GAP4 = 12.9853 eV DEX 4.5800 eV 3.9699 eV −7.8886 eV −8.7485 eV GAP1 = 11.8585 eV GAP2 = 12.4686 eV GAP3 = 12.4479 eV GAP4 = 13.3285 eV Molecules 2018, 23, x 9 of 17 2.3. Molecular Orbitals and Characteristic of Antioxidant Ability Table 3 shows data and representations of the molecular orbitals, GAP values (energy variation between frontier orbitals) between all the energy states GAP 1 , GAP 2 , GAP 3 and GAP 4 . The molecular orbitals LUMO and LUMO +1 have a direct relation with the electron affinity of a molecule, which is related to susceptibility to nucleophilic attacks [37]. The values of LUMO and LUMO+1 for Z91 and Z20 were close to those of the control molecules. This show that Z91 and Z20 are less susceptible to attack by nucleophiles. The values of LUMO and LUMO+1 were positively correlated with the Ki values. Only CP450 receptors (0.91 for LUMO and 0.88 for LUMO+1) and MP (0.54 for LUMO and 0.60 for LUMO+1) showed significant correlation values, revealing the tendency of the Ki values to decrease with the reduction of LUMO and LUMO+1 values for these receptors. Table 3. Representations of the molecular orbitals and their GAP values (energy variation between frontier orbitals—HOMO and LUMO) between all energy states ( [a] GAP 1 , [b] GAP2, [c] GAP3 and [d] GAP4) of the Z20, Z91, 5-fluorouracil (FLU), melatonin (MEL) and dextromethorphan (DEX). [a] GAP1 = LUMO − HOMO; [b] GAP2 = LUMO+1 – HOMO; [c] GAP3 =LUMO – HOMO-1; [d] GAP4 = LUMO+1 – HOMO-1. Significant correlation values with Ki were obtained for molecular orbital data (see Table 4). The HOMO orbital showed a significant correlation with Ki only in the MP receptor (0.47) and the # LUMO+1 LUMO HOMO HOMO-1 Z20 4.3201 eV 2.3282 eV −8.2834 eV −9.4429 eV GAP1 = 10.6116 eV GAP2 = 12.6035 eV GAP3 = 11.7711 eV GAP4 = 13.7630 eV Z91 4.3397 eV 2.3429 eV −8.2535 eV −9.4143 eV GAP1 = 10.5964 eV GAP2 = 12.5932 eV GAP3 = 11.7572 eV GAP4 = 13.7540 eV FLU 4.9438 eV 2.7592 eV −9.9485 eV −12.0353 eV GAP1 = 12.7077 eV GAP2 = 14.8923 eV GAP3 = 14.7945 eV GAP4 = 16.9791 eV MEL 5.2681 eV 3.5807 eV −7.3814 eV −7.6992 eV GAP1 = 10.9621 eV GAP2 = 12.6495 eV GAP3 = 11.2799 eV GAP4 = 12.9853 eV DEX 4.5800 eV 3.9699 eV −7.8886 eV −8.7485 eV GAP1 = 11.8585 eV GAP2 = 12.4686 eV GAP3 = 12.4479 eV GAP4 = 13.3285 eV Molecules 2018, 23, x 9 of 17 2.3. Molecular Orbitals and Characteristic of Antioxidant Ability Table 3 shows data and representations of the molecular orbitals, GAP values (energy variation between frontier orbitals) between all the energy states GAP 1 , GAP 2 , GAP 3 and GAP 4 . The molecular orbitals LUMO and LUMO +1 have a direct relation with the electron affinity of a molecule, which is related to susceptibility to nucleophilic attacks [37]. The values of LUMO and LUMO+1 for Z91 and Z20 were close to those of the control molecules. This show that Z91 and Z20 are less susceptible to attack by nucleophiles. The values of LUMO and LUMO+1 were positively correlated with the Ki values. Only CP450 receptors (0.91 for LUMO and 0.88 for LUMO+1) and MP (0.54 for LUMO and 0.60 for LUMO+1) showed significant correlation values, revealing the tendency of the Ki values to decrease with the reduction of LUMO and LUMO+1 values for these receptors. Table 3. Representations of the molecular orbitals and their GAP values (energy variation between frontier orbitals—HOMO and LUMO) between all energy states ( [a] GAP 1 , [b] GAP2, [c] GAP3 and [d] GAP4) of the Z20, Z91, 5-fluorouracil (FLU), melatonin (MEL) and dextromethorphan (DEX). [a] GAP1 = LUMO − HOMO; [b] GAP2 = LUMO+1 – HOMO; [c] GAP3 =LUMO – HOMO-1; [d] GAP4 = LUMO+1 – HOMO-1. Significant correlation values with Ki were obtained for molecular orbital data (see Table 4). The HOMO orbital showed a significant correlation with Ki only in the MP receptor (0.47) and the # LUMO+1 LUMO HOMO HOMO-1 Z20 4.3201 eV 2.3282 eV −8.2834 eV −9.4429 eV GAP1 = 10.6116 eV GAP2 = 12.6035 eV GAP3 = 11.7711 eV GAP4 = 13.7630 eV Z91 4.3397 eV 2.3429 eV −8.2535 eV −9.4143 eV GAP1 = 10.5964 eV GAP2 = 12.5932 eV GAP3 = 11.7572 eV GAP4 = 13.7540 eV FLU 4.9438 eV 2.7592 eV −9.9485 eV −12.0353 eV GAP1 = 12.7077 eV GAP2 = 14.8923 eV GAP3 = 14.7945 eV GAP4 = 16.9791 eV MEL 5.2681 eV 3.5807 eV −7.3814 eV −7.6992 eV GAP1 = 10.9621 eV GAP2 = 12.6495 eV GAP3 = 11.2799 eV GAP4 = 12.9853 eV DEX 4.5800 eV 3.9699 eV −7.8886 eV −8.7485 eV GAP1 = 11.8585 eV GAP2 = 12.4686 eV GAP3 = 12.4479 eV GAP4 = 13.3285 eV GAP1 = 10.9621 eV GAP2 = 12.6495 eV GAP3 = 11.2799 eV GAP4 = 12.9853 eV DEX Molecules 2018, 23, x 9 of 17 2.3. Molecular Orbitals and Characteristic of Antioxidant Ability Table 3 shows data and representations of the molecular orbitals, GAP values (energy variation between frontier orbitals) between all the energy states GAP 1 , GAP 2 , GAP 3 and GAP 4 . The molecular orbitals LUMO and LUMO +1 have a direct relation with the electron affinity of a molecule, which is related to susceptibility to nucleophilic attacks [37]. The values of LUMO and LUMO+1 for Z91 and Z20 were close to those of the control molecules. This show that Z91 and Z20 are less susceptible to attack by nucleophiles. The values of LUMO and LUMO+1 were positively correlated with the Ki values. Only CP450 receptors (0.91 for LUMO and 0.88 for LUMO+1) and MP (0.54 for LUMO and 0.60 for LUMO+1) showed significant correlation values, revealing the tendency of the Ki values to decrease with the reduction of LUMO and LUMO+1 values for these receptors. Table 3. Representations of the molecular orbitals and their GAP values (energy variation between frontier orbitals—HOMO and LUMO) between all energy states ( [a] GAP 1 , [b] GAP2, [c] GAP3 and [d] GAP4) of the Z20, Z91, 5-fluorouracil (FLU), melatonin (MEL) and dextromethorphan (DEX). [a] GAP1 = LUMO − HOMO; [b] GAP2 = LUMO+1 – HOMO; [c] GAP3 =LUMO – HOMO-1; [d] GAP4 = LUMO+1 – HOMO-1. Significant correlation values with Ki were obtained for molecular orbital data (see Table 4). The HOMO orbital showed a significant correlation with Ki only in the MP receptor (0.47) and the # LUMO+1 LUMO HOMO HOMO-1 Z20 4.3201 eV 2.3282 eV −8.2834 eV −9.4429 eV GAP1 = 10.6116 eV GAP2 = 12.6035 eV GAP3 = 11.7711 eV GAP4 = 13.7630 eV Z91 4.3397 eV 2.3429 eV −8.2535 eV −9.4143 eV GAP1 = 10.5964 eV GAP2 = 12.5932 eV GAP3 = 11.7572 eV GAP4 = 13.7540 eV FLU 4.9438 eV 2.7592 eV −9.9485 eV −12.0353 eV GAP1 = 12.7077 eV GAP2 = 14.8923 eV GAP3 = 14.7945 eV GAP4 = 16.9791 eV MEL 5.2681 eV 3.5807 eV −7.3814 eV −7.6992 eV GAP1 = 10.9621 eV GAP2 = 12.6495 eV GAP3 = 11.2799 eV GAP4 = 12.9853 eV DEX 4.5800 eV 3.9699 eV −7.8886 eV −8.7485 eV GAP1 = 11.8585 eV GAP2 = 12.4686 eV GAP3 = 12.4479 eV GAP4 = 13.3285 eV Molecules 2018, 23, x 9 of 17 2.3. Molecular Orbitals and Characteristic of Antioxidant Ability Table 3 shows data and representations of the molecular orbitals, GAP values (energy variation between frontier orbitals) between all the energy states GAP 1 , GAP 2 , GAP 3 and GAP 4 . The molecular orbitals LUMO and LUMO +1 have a direct relation with the electron affinity of a molecule, which is related to susceptibility to nucleophilic attacks [37]. The values of LUMO and LUMO+1 for Z91 and Z20 were close to those of the control molecules. This show that Z91 and Z20 are less susceptible to attack by nucleophiles. The values of LUMO and LUMO+1 were positively correlated with the Ki values. Only CP450 receptors (0.91 for LUMO and 0.88 for LUMO+1) and MP (0.54 for LUMO and 0.60 for LUMO+1) showed significant correlation values, revealing the tendency of the Ki values to decrease with the reduction of LUMO and LUMO+1 values for these receptors. Table 3. Representations of the molecular orbitals and their GAP values (energy variation between frontier orbitals—HOMO and LUMO) between all energy states ( [a] GAP 1 , [b] GAP2, [c] GAP3 and [d] GAP4) of the Z20, Z91, 5-fluorouracil (FLU), melatonin (MEL) and dextromethorphan (DEX). [a] GAP1 = LUMO − HOMO; [b] GAP2 = LUMO+1 – HOMO; [c] GAP3 =LUMO – HOMO-1; [d] GAP4 = LUMO+1 – HOMO-1. Significant correlation values with Ki were obtained for molecular orbital data (see Table 4). The HOMO orbital showed a significant correlation with Ki only in the MP receptor (0.47) and the # LUMO+1 LUMO HOMO HOMO-1 Z20 4.3201 eV 2.3282 eV −8.2834 eV −9.4429 eV GAP1 = 10.6116 eV GAP2 = 12.6035 eV GAP3 = 11.7711 eV GAP4 = 13.7630 eV Z91 4.3397 eV 2.3429 eV −8.2535 eV −9.4143 eV GAP1 = 10.5964 eV GAP2 = 12.5932 eV GAP3 = 11.7572 eV GAP4 = 13.7540 eV FLU 4.9438 eV 2.7592 eV −9.9485 eV −12.0353 eV GAP1 = 12.7077 eV GAP2 = 14.8923 eV GAP3 = 14.7945 eV GAP4 = 16.9791 eV MEL 5.2681 eV 3.5807 eV −7.3814 eV −7.6992 eV GAP1 = 10.9621 eV GAP2 = 12.6495 eV GAP3 = 11.2799 eV GAP4 = 12.9853 eV DEX 4.5800 eV 3.9699 eV −7.8886 eV −8.7485 eV GAP1 = 11.8585 eV GAP2 = 12.4686 eV GAP3 = 12.4479 eV GAP4 = 13.3285 eV Molecules 2018, 23, x 9 of 17 2.3. Molecular Orbitals and Characteristic of Antioxidant Ability Table 3 shows data and representations of the molecular orbitals, GAP values (energy variation between frontier orbitals) between all the energy states GAP 1 , GAP 2 , GAP 3 and GAP 4 . The molecular orbitals LUMO and LUMO +1 have a direct relation with the electron affinity of a molecule, which is related to susceptibility to nucleophilic attacks [37]. The values of LUMO and LUMO+1 for Z91 and Z20 were close to those of the control molecules. This show that Z91 and Z20 are less susceptible to attack by nucleophiles. The values of LUMO and LUMO+1 were positively correlated with the Ki values. Only CP450 receptors (0.91 for LUMO and 0.88 for LUMO+1) and MP (0.54 for LUMO and 0.60 for LUMO+1) showed significant correlation values, revealing the tendency of the Ki values to decrease with the reduction of LUMO and LUMO+1 values for these receptors. Table 3. Representations of the molecular orbitals and their GAP values (energy variation between frontier orbitals—HOMO and LUMO) between all energy states ( [a] GAP 1 , [b] GAP2, [c] GAP3 and [d] GAP4) of the Z20, Z91, 5-fluorouracil (FLU), melatonin (MEL) and dextromethorphan (DEX). [a] GAP1 = LUMO − HOMO; [b] GAP2 = LUMO+1 – HOMO; [c] GAP3 =LUMO – HOMO-1; [d] GAP4 = LUMO+1 – HOMO-1. Significant correlation values with Ki were obtained for molecular orbital data (see Table 4). The HOMO orbital showed a significant correlation with Ki only in the MP receptor (0.47) and the # LUMO+1 LUMO HOMO HOMO-1 Z20 4.3201 eV 2.3282 eV −8.2834 eV −9.4429 eV GAP1 = 10.6116 eV GAP2 = 12.6035 eV GAP3 = 11.7711 eV GAP4 = 13.7630 eV Z91 4.3397 eV 2.3429 eV −8.2535 eV −9.4143 eV GAP1 = 10.5964 eV GAP2 = 12.5932 eV GAP3 = 11.7572 eV GAP4 = 13.7540 eV FLU 4.9438 eV 2.7592 eV −9.9485 eV −12.0353 eV GAP1 = 12.7077 eV GAP2 = 14.8923 eV GAP3 = 14.7945 eV GAP4 = 16.9791 eV MEL 5.2681 eV 3.5807 eV −7.3814 eV −7.6992 eV GAP1 = 10.9621 eV GAP2 = 12.6495 eV GAP3 = 11.2799 eV GAP4 = 12.9853 eV DEX 4.5800 eV 3.9699 eV −7.8886 eV −8.7485 eV GAP1 = 11.8585 eV GAP2 = 12.4686 eV GAP3 = 12.4479 eV GAP4 = 13.3285 eV Molecules 2018, 23, x 9 of 17 2.3. Molecular Orbitals and Characteristic of Antioxidant Ability Table 3 shows data and representations of the molecular orbitals, GAP values (energy variation between frontier orbitals) between all the energy states GAP 1 , GAP 2 , GAP 3 and GAP 4 . The molecular orbitals LUMO and LUMO +1 have a direct relation with the electron affinity of a molecule, which is related to susceptibility to nucleophilic attacks [37]. The values of LUMO and LUMO+1 for Z91 and Z20 were close to those of the control molecules. This show that Z91 and Z20 are less susceptible to attack by nucleophiles. The values of LUMO and LUMO+1 were positively correlated with the Ki values. Only CP450 receptors (0.91 for LUMO and 0.88 for LUMO+1) and MP (0.54 for LUMO and 0.60 for LUMO+1) showed significant correlation values, revealing the tendency of the Ki values to decrease with the reduction of LUMO and LUMO+1 values for these receptors. Table 3. Representations of the molecular orbitals and their GAP values (energy variation between frontier orbitals—HOMO and LUMO) between all energy states ( [a] GAP 1 , [b] GAP2, [c] GAP3 and [d] GAP4) of the Z20, Z91, 5-fluorouracil (FLU), melatonin (MEL) and dextromethorphan (DEX). [a] GAP1 = LUMO − HOMO; [b] GAP2 = LUMO+1 – HOMO; [c] GAP3 =LUMO – HOMO-1; [d] GAP4 = LUMO+1 – HOMO-1. Significant correlation values with Ki were obtained for molecular orbital data (see Table 4). The HOMO orbital showed a significant correlation with Ki only in the MP receptor (0.47) and the # LUMO+1 LUMO HOMO HOMO-1 Z20 4.3201 eV 2.3282 eV −8.2834 eV −9.4429 eV GAP1 = 10.6116 eV GAP2 = 12.6035 eV GAP3 = 11.7711 eV GAP4 = 13.7630 eV Z91 4.3397 eV 2.3429 eV −8.2535 eV −9.4143 eV GAP1 = 10.5964 eV GAP2 = 12.5932 eV GAP3 = 11.7572 eV GAP4 = 13.7540 eV FLU 4.9438 eV 2.7592 eV −9.9485 eV −12.0353 eV GAP1 = 12.7077 eV GAP2 = 14.8923 eV GAP3 = 14.7945 eV GAP4 = 16.9791 eV MEL 5.2681 eV 3.5807 eV −7.3814 eV −7.6992 eV GAP1 = 10.9621 eV GAP2 = 12.6495 eV GAP3 = 11.2799 eV GAP4 = 12.9853 eV DEX 4.5800 eV 3.9699 eV −7.8886 eV −8.7485 eV GAP1 = 11.8585 eV GAP2 = 12.4686 eV GAP3 = 12.4479 eV GAP4 = 13.3285 eV GAP1 = 11.8585 eV GAP2 = 12.4686 eV GAP3 = 12.4479 eV GAP4 = 13.3285 eV [a] GAP1 = LUMO − HOMO; [b] GAP2 = LUMO+1 − HOMO; [c] GAP3 =LUMO − HOMO-1; [d] GAP4 = LUMO+1 −HOMO-1. Molecules 2018,23, 2801 16 of 17 44. 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