Stepwise hydration of cellobiose by DFT methods: 1. Conformational and structural changes brought about by the addition of one to four water molecules

Previous density-functional theory (DFT) calculations found that the anti (or “flipped”) form of cellobiose (with the H1 and H4′ hydrogen atoms on opposite sides of the pseudo-plane formed by the sugar rings) is more stable in vacuo than the syn (or “normal”) conformation most often observed in crystalline- and solution-phase experiments. In order to understand the reason for this conformational preference, cellobiose-water complexes were optimized at the B3LYP/6-311++G∗∗ level of theory. Ten different anhydrous cellobiose structures were used as starting points, and the results of calculations on 30 monohydrates, 20 dihydrates, 12 trihydrates, and 5 tetrahydrates are presented. The syn form of the molecule was stabilized relative to the anti form as more water molecules were added, with the two conformers being approximately equal in stability at the dihydrate level. Addition of more than two water molecules further increased the relative stability of the syn conformer. One reason for the increase in stability of the syn form upon hydration is the ability of that conformational class to better accommodate a water molecule between the two rings. Changes in bond lengths, bond angles, and dihedral angles that occur due to the interactions with water molecules are described in detail. These hydration induced structural changes are largely localized near the water molecule(s), and the effects of the addition of subsequent water molecules can be predicted based upon the structure differences between the monohydrates and the corresponding anhydrous cellobiose conformers.