Conformational preferences in diglycosyl disulfides: NMR and molecular modeling studies

Accepted Manuscript Conformational Preferences in Diglycosyl Disulfides: Nmr and Molecular Mod‐ elling Studies Krisztina Fehér, Richard P. Matthews, Katalin E. Kövér, Kevin J. Naidoo, László Szilágyi PII: S0008-6215(11)00371-5 DOI: 10.1016/j.carres.2011.07.013 Reference: CAR 5864 To appear in: Carbohydrate Research Received Date: 13 May 2011 Revised Date: 5 July 2011 Accepted Date: 12 July 2011 Please cite this article as: Fehér, K., Matthews, R.P., Kövér, K.E., Naidoo, K.J., Szilágyi, L., Conformational Preferences in Diglycosyl Disulfides: Nmr and Molecular Modelling Studies, Carbohydrate Research (2011), doi: 10.1016/j.carres.2011.07.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. CONFORMATIONAL PREFERENCES IN DIGLYCOSYL DISULFIDES: NMR AND MOLECULAR MODELLING STUDIES Krisztina Fehér#a,b, Richard P. Matthews#c,d, Katalin E. Kövérb, Kevin J. Naidoo*c,d, László Szilágyi*a Departments of Organica and Inorganicb Chemistry, University of Debrecen , H-4010 Debrecen, Pf. 20., Hungary and Scientific Computing Research Unitc and Department of Chemistryd, University of Cape Town, Rondebosch 7701, South Africa Abstract The conformations of several 1 1’ diglycosyl disulfides were investigated by NMR and computational methods. Experimental data such as NOEs, proton-proton and proton-carbon-13 coupling constants, measured for solutions in DMSO, are in good agreement with values obtained by MD simulations in explicit DMSO. The disulfide torsion angles (C1-S-S-C1’) preferentially sample values close to either +90o or –90o (+g or –g) and appear as the main metric that determines the conformational behavior of these glycomimetics. There is more conformational freedom around the C1-S and C1’-S’ bonds (Φ and Ω torsions, respectively) and population cluster analysis allowed to identify up to four allowed conformational regions for each of the +g or –g forms. Population analysis of the hydroxylic group rotamers, based on protonproton and proton-carbon-13 couplings as well as on calculated hydrogen bonding statistics, did not reveal any significant intramolecular hydrogen bonds in DMSO solution. *Authors to whom correspondence should be addressed e-mail: [email protected] e-mail: [email protected] # These authors contributed equally to computational analyses. Keywords: diglycosyl disulfides, conformation, MD simulation, hydroxylic rotamer populations, NMR coupling constants, NOEs 1. Introduction Making and breaking of the glycosidic bond is fundamental to carbohydrate chemistry and biology. Natural glycosides are hydrolyzed by glycosidase enzymes, a reaction often implicated in pathological processes. Resistance to glycoside hydrolases is most efficiently achieved by manipulating the glycosidic linkage; it is therefore of prime importance to investigate the physicochemical properties of this bond. Natural glycosidic linkages usually comprise two bonds (C-X-C) between the sugar ring and an aglycone, or between two sugar rings, with the bridging atom being an oxygen (X=O) in the majority of cases. Three-bond interglycosidic linkages (3BIGLs, C-X-Y-C) with X=C and Y=O are not uncommon either but 3BIGLs with two heteroatoms (X,Y = O, N, S) were unknown in Nature until the discovery of the esperamicin-calicheamicin group of antitumor antibiotics. A striking feature of these structures is the unusual –N-Oglycosidic bond in the oligosaccharide parts of the molecules (for a review, see:1) NMR2 and X-ray3 data together with force field calculations4 highlighted the importance of the conformation around the –N-Olinkage to ensure optimal binding to DNA for subsequent cleavage.5,6 Introduction of a disulfide motif (X,Y = S) to connect two different monosaccharide units led to a novel class of disaccharide mimics7-10 characterized by another two heteroatom-3BIGL (for a review, see Szilágyi et. al11). Some neoglycoproteins represent further examples of interesting hybrid structures in which glycosyl units are attached to proteins through S-S linkages.12-14 Symmetric diglycosyl disulfides were shown to bind specifically to the plant lectin Concanavalin A15, furthermore, inhibitory activities against an endogenous lectin were detected in vivo on human tumor cell lines.16 Based on these results disulfide-linked sugar derivatives were suggested “as new substance platform for lectin-directed drug design”.16 We have recently described specific binding of oligovalent aromatic mannosyl disulfide derivatives to Concanavalin A.17 In order to gain insight into biomolecular interactions, structural characterization of the molecules involved is necessary. Experimental and theoretical studies on sugar disulfides have been limited thus far. The crystal and molecular structures of symmetric18,19 and nonsymmetric diglycosyl disulfide derivatives20,21 and some alkyl-glycosyl disulfide structures22,19 have been published. Recently we investigated the chiroptical properties of diglycosyl disulfides and - selenides in solution and in the solid state.23 Previously the conformations of a 1Æ4 disulfide disaccharide were estimated in solution on the basis of qualitative NOEs and simple MO considerations,7 while molecular dynamics simulations without experimental restraints were reported for the symmetric β(1Æ1)digalactosyl-disulfide.16 High resolution NMR spectroscopy holds the most promise for experimental studies of carbohydrate conformations and dynamic properties in solution (for a recent review, see:24). Vicinal coupling constants proved valuable to convey information on the conformation at the glycosidic dihedrals25-27 or in determining the distribution of hydroxymethyl rotamers.28-30 Nuclear Overhauser effects (NOEs) are useful to estimate internuclear distances but interpretation of the data is complicated by conformational averaging on the NMR timescale and the limited number of observable NOEs between monosaccharide units.31,32 Supplementing experimental data with molecular dynamics (MD) simulations provides an efficient approach to handle multiple conformations33,34,27,35,36,31 and to explore the flexibility of glycosidic linkages. Here we use an integrated experimental and computational approach to gain insight into the conformational behavior of a series of diglycosyl-disulfides. Specifically we report our results on the disaccharide mimics D-Glcp-βS(1Æ1’)Sβ-D-Glcp, D-Glcp-βS(1Æ1’)Sβ-D-Galp, D-GlcpβS(1Æ1’)Sβ-D-Manp, and D-Glcp-βS(1Æ1’)Sβ-D-GlcNAcp for solutions in DMSO. These are referred to as GSSG, GSSGa and GSSM and GSSGN (Figure 1), respectively to simplify the discussion below. R 1 R 2 R 3 R 4 GSSG H OH H OH GSSGa H OH OH H GSSM OH H H OH GSSGN H NHAc H OH Figure 1. Schematic of the disaccharides studied and definition of the torsion angles about the disulfide interglycosidic linkage. 2. Materials and methods The compounds investigated in the present study have been synthesized according to published procedures.10 2.1 NMR measurements NMR experiments were run on a Bruker Avance DRX-500 spectrometer. 10-15 mg of the samples were dissolved in DMSO-d6 and the probe temperature was set to 300K. 2D NOE (NOESY) and rotating frame ROE (ROESY) spectra in phase sensitive mode (TPPI) were obtained using standard Bruker pulse programs; the mixing time being 300 ms in both experiments for each compound. A CW spinlock field of 3.57 kHz strength was used for the ROESY experiments. HOHAHA effects37,38 and J-relayed cross peaks39,40 were identified by repeating the experiment using a different offset frequency for the spinlock field.41 The raw datasets typically consisted of 2K 512 complex data points. The cross peak intensities were determined by volume integration from the baseplane corrected spectra. The individually assigned cross peaks were checked for the absence of the artefacts above before converting them into distances using the Isolated Spin Pair Approximation (ISPA).42 The distances between 1,3 and 1,5 diaxial protons within the glycopyranosyl rings with 4C1 chair geometry43,44 were used for calibration of the integrals. Long-range proton-carbon coupling constants were determined using a sensitivity enhanced, gradient selected 13C-filtered TOCSY pulse sequence called gs-HETLOC45 and HSQMBC experiments.46,47 Theoretical values for homoand heteronuclear vicinal couplings involving hydroxyl protons were calculated from MD-simulated torsion angles using the Karplus type equations below,48,49 2.0cos5.1cos4.10 2 , 3+−= ϕϕ OHH J (1) 102.0cos585.0cos494.5 23 +−= ϑϑ HOCC J (2) where φ is defined as Hi-Ci-O-Hi and ϑ as Ci-1-Ci-O-Hi. Populations of the hydroxyl rotamers around the C-O bond were estimated from the measured 3JH,OH and 3JHOCC coupling constants49 and compared with calculated values (see Results and Discussion). 2.2 Computational 2.2.1 Adiabatic maps The energy landscapes for each disaccharide were explored as a function of the three torsion angles that describe the conformation about the disulfide linkage. They are defined as: Φ = H1 – C1 – S1 – S1’, Ψ = C1– S1 – S1’ – C1’, and Ω = S1 – S1’ – C1’ – H1’ (see Fig.1). Adiabatic maps were constructed in three dimensional ( , , ) space using the simulated annealing algorithm50 as described previously for the three-bond glycosidic linkage in isomaltose36 and panose.35 The disaccharides were modeled using the CSFF force field51 with the addition of CSFF consistent parameters for the SS-linkage.52 2.2.2 Molecular Dynamics simulations Based on the three-dimensional adiabatic maps, low lying minima were chosen as starting points for simulations in explicit DMSO. The molecular dynamics program CHARMM33b53 was used for all the simulations. DMSO MD simulations were conducted using the Strader et al. model for DMSO.54 The dynamics were run in a cubic box with sides 41.7262 Å applying periodic boundary conditions. The disaccharides were solvated with 612 DMSO molecules. The overlapping DMSO molecules within a heavy atom distance of 3.5 Å from the solute were subsequently removed. After an initial 1 ns equilibration the MD simulations were extended for a further 10 ns for the conformations that were most consistent with experimental NOE data, while shorter, 5 ns simulations were run for the remaining conformations. The simulations were carried out using the leapfrog Verlet integrator and implementation of the isothermal-isobaric ensemble (NPT) where the pressure and temperature are kept constant (P = 1 bar, T = 300 K) by making use of the Langevin piston method.55 Data for all respective configurations were stored at intervals of 0.05 ps and 0.5 ps for the 10 ns and 5 ns long simulations, respectively. 2.2.3 Computational Analysis Population cluster analysis (PCA) was performed using the ART2’ adaptive analogue pattern recognition scheme.56,57 A cluster radius value58 was used to prevent false clusters from arising. In order to measure the occurrence of intraand intermolecular hydrogen bonds during the MD simulation hydrogen bonding statistics were calculated. We used a geometric definition of a maximum distance criterion of 2.4 Å between the acceptor oxygen and the hydrogen along with an angle of not less than 100o for (donor oxygen)-hydrogen-(acceptor oxygen) arrangement. 3. Results and discussion 3.1 Conformational analysis of the interglycosidic torsion angles based on adiabatic maps and MD simulations in explicit DMSO The three dimensional adiabatic energy surface revealed local minima primarily in two main areas characterized by −90o and +90o along the Ψ angle for all disulfide disaccharides. Therefore the conformational space is discussed in terms of these two low energy regions separately. Conformations close to Ψ = −90o and +90o minima are designated as –g and +g conformations, respectively. The potential energy surfaces shown in Figs. 2 and 3 were obtained by slicing along the Ψ = −90° and Ψ = +90° plane of the three dimensional adiabatic surfaces and contouring at energies of 2 kcal.mol-1 from the lowest minimum and up to 12 kcal.mol-1. Within these regions several local minima exist for each of the disaccharides, these are identified by letters A to D in the –g conformation (Fig. 2) and E to G in the +g conformation (Fig. 3; for a detailed listing of the data, see Supplementary Table S1). Figure 2. Representative two-dimensional sections from the (a) GSSG, (b) GSSGa, (c) GSSM and (d) GSSGN adiabatic maps for Ψ = −90°. The energy is contoured in increments of 2 kcal.mol-1 above the local minimum. Figure 3. Representative two-dimensional sections from the (a) GSSG, (b) GSSGa, (c) GSSM and (d) GSSGN adiabatic maps for Ψ = +90°. The energy is contoured in increments of 2 kcal.mol-1 above the local minimum. The results of the MD simulations in explicit DMSO are displayed in Figs. 4 and 5 by depicting cluster distributions in the Φ/Ω space. Population averaged angles of the clusters of the MD simulation are summarized in Table 1 along with the torsion angles of their starting structures. Figure 4. Representations of the clusters arising from MD trajectories calculated in DMSO and started in the −g conformation. Each symbol identifies a cluster obtained by PCA of the MD run started from the adiabatic minima A-D labeled with symbols ,, S and { respectively. Conformational regions are denoted by roman numerals in accordance with Table 1. Data are shown for the –g conformations of GSSG (a), GSSGa (b), GSSM (c), and GSSGN (d). Figure 5. Representations of the clusters arising from MD trajectories calculated in DMSO and started in the +g conformation. Each symbol identifies a cluster obtained by PCA of the MD run started from the adiabatic minima A-D labeled with symbols U, , and { respectively. Conformational regions are denoted by roman numerals in accordance with Table 1. Data are shown for the +g conformations of GSSG (a), GSSGa (b), GSSM (c), and GSSGN (d). For GSSG the lowest energy minimum is A in the –g conformation followed closely by minimum E with relative potential energy of 0.256 kcal.mol-1 in the +g conformation as shown in Table S1. All other minima have energies more than 1 kcal.mol-1 higher than the global minimum. MD simulations started from each of the adiabatic minima resulted in four conformational regions listed in Table 1 (last column). Inspection of the data shows that for GSSG in the –g conformation minima A and D were quasi conserved in the MD simulations yielding cluster groups II and III, while minimum C shifted along the angle to a region, designated as I as shown in Fig. 4a. In contrast, all simulations in the +g conformation resulted in a single region centred close to minimum G, named as region V as depicted in Fig. 5a. The population-averaged Ψ values are slightly less than the ideal +/−90o for all clusters except for region I, which is characterized by Ψ = −100o (Table 1). For GSSGa the global energy minimum is state A in the -g conformation as shown in Table S1, while the second lowest energy minimum is well B with 0.98 kcal.mol-1, also found in the –g conformation. The MD simulations started from the different adiabatic minima yielded altogether six regions listed in Table 1. Four regions of the conformational space were sampled in the –g conformation, corresponding to the areas around minima A, C, D yielding conformational groups III, II, I and an additional new region, conformational area IV was found, which emerged from the simulation started from minimum B as shown in Fig. 4b. In the +g conformational state the MD simulations sampled conformations in one broad region encompassing all minima E, F and G. By comparison, +g conformations of the other derivatives could be clustered along Ω into the two regions designated as V and VI, as seen in Fig. 5b. The population averaged Ψ angles are slightly less than the ideal +/−90o for all clusters, while region IV displays a distinctly different Ψ angle (−107o). For GSSM the global energy minimum is in the +g conformation E, though it is closely followed by minimum A in the -g conformation, which has only 0.2 kcal.mol-1 higher energy than E as shown in Table S1. There are yet another two minima which are relatively close to the global minimum: state B with 0.775 kcal.mol-1 and F with 0.997 kcal.mol-1. The MD simulations started from the various minima explored only four regions of the conformational space listed in Table 1. In the –g conformation only areas around minima D and C were verified by the MD simulation resulting in conformational regions I and II as shown in Fig. 4c. In the +g conformation a region around minimum E designated as area VI and clusters arising from the simulation started from minimum G, named as conformational group VII, were identified as shown in Fig. 5c. All resulting regions have similar Ψ values of somewhat smaller than +/−90o with the exception of the cluster starting from G (105o). For GSSGN the global energy minimum is A in the -g conformation with all other conformations having significantly higher potential energy as shown in Table S1. The MD simulations for GSSGN produced three distinct regions in both of the –g (I, II and III, Fig. 4d) and +g ( V, VI and VIII, Fig. 4b) conformational states as listed in Table 1. All six regions displayed very similar Ψ values of somewhat less than the ideal +/−90o. Table 1 Torsion angles of minimum energy starting structures for MD simulations yielding conformational regions with population averaged torsion angles. MD population averaged Adiabatic map minima Φ Ψ Ω Φ Ψ Ω Conformational regions GSSG A -170 -90 0 -166 -82 38 III B 60 -90 -40 182 -85 19 III C 50 -90 50 -2 -100 -21 I D 30 -90 -180 30 -86 179 II E 60 90 -50 -5 84 46 V F 60 90 -50 -22 86 55 V G -40 90 40 -22 85 46 V GSSGa A 180 -90 20 -171 -83 -4 III B -140 -90 -160 -50 -107 171 IV C 30 -90 170 39 -78 164 II D 30 -90 30 35 -88 38 I D -178 -90 35 III E -40 90 40 -8 81 33 V E 21 86 -22 VI F 80 90 -20 -35 90 44 V G 40 90 -30 -1 87 -39 VI GSSM A -150 -90 170 30 -81 181 II B 180 -90 -10 21 -91 59 I C 30 -90 150 -3 -86 194 II D 50 -90 30 45 -86 43 I E 60 90 -30 28 82 -56 VI F -170 90 20 42 79 -62 VI G -20 90 50 174 105 52 VII GSSGN A 180 -90 30 -164 -85 13 III B 180 -90 0 -27 -86 -20 I C -30 -90 20 13 -90 28 I D 40 -90 170 50 -89 178 II E 70 90 -20 37 85 -53 VI F -20 90 50 18 84 27 V G -40 90 170 12 90 164 VIII The adiabatic maps gave an overall impression of the minimum energy conformations, but some of these regions disappeared or shifted to a slightly different area of the potential energy surface during the MD simulations. The differences between the adiabatic maps and the space sampled by MD may be attributed to solvation effects in the MD simulation. Overall the number of final conformational regions were reduced in comparison with the number of adiabatic minima. The MD simulations confirmed that the Ψ angle preferences were ca. +/−90o. The distributions of Ψ values in the MD simulations were restricted to a comparatively narrow range Good correlation was observed between experimental and calculated values for all 1H-1H coupling constants. The majority of the calculated 3JHOCC couplings also show reasonable agreement with measured values. Differences exceeding +/- 1Hz may be due to partial exchange of OH-protons on the experimental side and/or substituent effects not properly taken into account in eq. (2) in the calculations. 3JHC coupling constants are known to be notoriously sensitive to substituent effects (see63 and references cited therein). A graphical representation (using GSSGa as an illustration) of the correlation between experimental and calculated values is shown in Figure 8. Fig. 8. Correlation between experimental and calculated coupling constants for GSSGa (cf. Tables 4 & 5). Color coding and symbols: *, blue: 3JHOCH measured; o, red: 3JHOCH calculated; •, green: 3JHOCC measured; o, black: 3JHOCC calculated. Error bars are added in matching colours. According to the Karplus equations (1) & (2) coupling constants of the order of ~ 5.4 Hz and ~ 2.8 Hz, for 3JHOCH and 3JHOCC, respectively, represent rotation-averaged values. The data in Tables 4 and 5 are close to these values and therefore indicate quasi free rotation around the C-O bond for all hydroxyl groups investigated. When both couplings are available for a particular OH further information can be deduced, namely, populations of each of the OH rotameric states can be estimated in terms of antiperiplanar (ap) and synclinal (sc) conformations around the C-O bond.49 Population analyses of the conformational preferences of the same hydroxyl groups were performed for both the +g and –g simulations by binning the sampled rotamers in increments of 5o. Rotamers within 5o of any transitions, i.e. +sc to -sc, etc. were excluded from the final summation. OH rotameric populations that were estimated from NMR coupling constants show good correlation with values obtained from MD simulations (Table 6). Rotameric stabilization may be promoted by stereo-electronic effects and/or hydrogen bonding. Calculated H-bonding statistics presented above are, however, ruling out the occurrence of persistent hydrogen bonds in DMSO solution. This is (also) in line with experimental and theoretical results demonstrating the disruption of internal hydrogen bonds in solvents with increased polarity such as water or DMSO.60-62 The data in Table 6 indicate no preferred rotameric state for the majority of the OH groups in GSSGa, GSSM and GSSGN. Low percentage of the ap rotamers for Glu-OH3 and Gal-OH4 in GSSGa and one of the sc rotamers for GlcNAc-OH3 in GSSGN were found, however, by experiment and calculation alike (Table 6). According to the considerations above this restriction of the rotameric freedom may be due stereo-electronic effects rather than engagement into internal hydrogen bonds of these particular OH groups. Table 6. NMR and MD rotamer population distributions of OH groups Compound Group NMR measured rotamers (%)* MD calculated rotamers (%)Ŧ P(-sc) P(ap) P(+sc) P(-sc) P(ap) P(+sc) GSSGa Glu-OH2 36 37 17 32 40 21 Glu-OH3 17 9 28 26 4 49 Glu-OH4 33 23 37 35 10 45 Glu-OH6 36 29 n.a. 45 40 13 Gal-OH2 40 43 28 40 40 18 Gal-OH3 37 17 37 36 19 40 Gal-OH4 60 9 29 60 18 20 GSSM Glu-OH2 38 48 20 37 42 20 Glu-OH3 35 27 28 28 6 47 Glu-OH4 34 27 44 24 22 50 Glu-OH6 37 23 n.a. 46 42 8 Man-OH2 13 n.a. 34 11 53 34 Man-OH3 23 33 34 11 25 44 Man-OH4 30 23 45 18 14 50 Man-OH6 37 23 n.a. 45 38 15 GSSGN Glu-OH2 38 35 n.a. 40 40 17 Glu-OH3 n.a. n.a. n.a. 55 42 1 Glu-OH4 29 30 41 30 15 50 Glu-OH6 31 23 n.a. 44 41 5 GlcNAcOH3 0 45 33 3 37 57 *As determined from three-bond proton-proton- (3JH,OH) and proton-carbon (3JHOCC) coupling constants (see text). The +sc, -sc and ap notations refer to the synclinal or antiperiplanar orientation of proton OH(i) with respect to C(i-1). Populations P(n) are approximate (+/−10%) values. ŦMD rotamer populations calculated for the +g/-g average conformation. 4. Conclusion In summary, experimental NMR data such as NOEs, proton-proton and proton-carbon-13 coupling constants supplemented with MD calculations in explicit DMSO have clearly indicated that the main metric to determine the conformations in 1 1’ diglycosyl disulfides is the disulfide torsion angle (C1-S-S-C1’) which preferentially samples values close to either +90o or –90o (+g or –g). Significantly more conformational freedom was observed around the C1-S and C1’-S’ bonds (Φ and Ω torsions, respectively) and population cluster analysis (PCA) allowed to identify up to four allowed conformational regions for each of the +g or –g forms. Regarding conformational similarities vs. differences between the members of the current panel of diglycosyl disulfide structures it can be stated in general that substituent effects (steric or electronic) of groups close to the disulfide bridge have larger impact on the conformational distributions than changes at remote positions. Visual comparison of adiabatic maps and MD simulations both show the highest conformational similarity of GSSG to GSSGa, less similarity to GSSGN and even less to GSSM. In particular, in the –g form conformational groups I, II and III are sampled by GSSG, GSSGa and GSSGN whereas only groups I and II are present for GSSM. In the +g form conformational group V, which is dominant for GSSG, is shared only by GSSGa and GSSGN, but is not present for GSSM. This is in line with the general trend stated above. The change of the 4-OH group from equatorial (GSSG) to axial (GSSGa) position is far away from the disulfide bridge and therefore does not result in large conformational differences. The 2-NAc group in GSSGN is closer to disulfide bridge, however, the substitution does not significantly affect the stereoelectronic charaterisitics of GSSGN compared to GSSG. The GSSM derivative features the largest difference compared to GSSG because the equatorial 2-OH group, next to the disulfide linkage, in the latter is changed for an axial one in the mannosyl unit of the former. Estimation of the hydroxylic group rotamer populations, based on calculated hydrogen bonding statistics and 1H-1H / 1H-13C J-coupling analysis, did not reveal any significant intramolecular hydrogen bonds in DMSO solution. Knowledge of the conformational preferences in this novel class of glycomimetics will certainly contribute to the design of further derivatives and provide insight into biologically relevant interactions such as binding to lectins15,17 or tumor cells.16 Acknowledgements This work is based upon research supported by the South African Research Chairs Initiative of the Department of Science and Technology and National Research Foundation to KJN and by the South African Hungarian collaboration programs (TéT DAK-1/02, ZA-26/2006 and ZA-20/2008) and by OTKA (NK-68578), TÁMOP-4.2.2-08/1/2008-0019 and TÁMOP-4.2.1./B-09/1/KONV2010-0007 grants in Hungary. References 1. Smith, A. L.; Nicolaou, K. C. J. Med. Chem. 1996, 39, 2103-2117. 2. Walker, S.; Valentine, K. G.; Kahne, D. J. Am. Chem. Soc. 1990, 112, 6428-6429. 3. Lee, M. D.; Dunne, T. S.; Chang, C. C.; Ellestad, G. A.; Siegel, M. M.; Morton, G. O.; McGahren, W. J.; Borders, D. B. J. Am. Chem. Soc. 1987, 109, 3466-3468. 4. 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Conformational preferences in diglycosyl disulfides: NMR and molecular modelling studies Krisztina Fehér, Richard P. Matthews, Katalin E. Kövér, Kevin J. Naidoo, László Szilágyi* The disulfide torsion angles (Ψ) sample values close to +90o or –90o (+g or –g) and up to four allowed conformational regions around Φ and Ω were identified for each of the Ψ values. HIGHLIGHTS Conformations of diglycosyl disulfides were studied by NMR and computations The disulfide torsion angles (Ψ) are close to +90o or –90o (+g or –g) Up to four conformational regions around Φ and Ω are allowed for each Ψ values No significant intramolecular hydrogen bonds were revealed in DMSO solution.