A DFT/ab initio study of hydrogen bonding and conformational preference in model cellobiose analogs using B3LYP/6-311++G**.

A series of beta-cellobiose analogs were studied at the B3LYP/6-311++G** level of theory to isolate and understand how the various electronic components of the beta-(1-->4)-linked disaccharide, cellobiose, contribute to the energetic stability of the molecule in vacuo. Previous studies on beta-cellobiose (see accompanying paper) showed that the most energetically stable conformation was that in which the dihedral angle phi (phi(H)) was 'flipped' by approximately 180 degrees relative to the 'normal' form. From our examination of eight sets of structures in which different combinations of functional hydroxyl and hydroxymethyl groups were removed, it was determined that only beta-cellobiose and one other analog (analog 7, beta-xylobioside), an analog in which both hydroxymethyl groups were removed but the exocyclic hydroxyl groups retained, can form a 'cooperative' hydrogen-bonding network. Only in these two molecules did we find continuous synergistic 'communication' through hydrogen bonding from one sugar moiety to the other. This 'cooperative' hydrogen bonding energetically stabilizes the 'flipped' conformation of beta-cellobiose and beta-xylobioside, while the other analogs studied were unable to form a 'cooperative' grouping of hydrogen bonds and thus were more stable in their 'normal' conformational state.