Conformational Properties of Methyl β-Maltoside and Methyl α- and β-Cellobioside Disaccharides

An investigation of the conformational properties of methyl beta-maltoside, methyl alpha-cellobioside, and methyl beta-cellobioside disaccharides using NMR spectroscopy and molecular dynamics (MD) techniques, is presented. Emphasis is placed on validation of a recently presented force field for hexopyranose disaccharides followed by elucidation of the conformational properties of two different types of glycosidic linkages, alpha-(1 -> 4) and beta-(1 -> 4). Both gas-phase and aqueous-phase simulations are performed to gain insight into the effect of solvent on the conformational properties. A number of transglycosidic J-coupling constants and proton-proton distances are calculated from the simulations and are used to identify the percent sampling of the three glycosidic conformations (syn, anti-phi, and anti-psi) and, in turn, describe the flexibility around the glycosidic linkage. The results show the force field to be in overall good agreement with experiment, although some very small limitations are evident. Subsequently, a thorough hydrogen bonding analysis is performed to obtain insights into the conformational properties of the disaccharides. In methyl beta-maltoside, competition between HO2'-O3 intramolecular hydrogen bonding and intermolecular hydrogen bonding of those groups with solvent leads to increased sampling of syn, anti-phi, and anti-psi conformations and better agreement with NMR J-coupling constants. In methyl alpha- and beta-cellobioside, O5' HO6 and HO2'-O3 hydrogen bonding interactions are in competition with intermolecular hydrogen bonding involving the solvent molecules. This competition leads to retention of the O5'-HO3 hydrogen bond and increased sampling of the syn region of the phi/psi map. Moreover, glycosidic torsions are correlated to the intramolecular hydrogen bonding occurring in the molecules. The present results verify that in the beta-(1 -> 4)-linkage intramolecular hydrogen bonding in the aqueous phase is due to the decreased ability of water to successfully compete for the O5' and HO3 hydrogen bonding moieties, in contrast to that occurring between the O5' and HO6 atoms in this alpha-(1 -> 4)-linkage.