Stepwise hydration of cellobiose by DFT methods: 2. Energy contributions to relative stabilities of cellobiose·(H2O)1-4 complexes

In the preceding paper, it was shown that the anti form of cellobiose, which is more stable in vacuo than the syn conformation, becomes less energetically favored as an increasing number of water molecules are complexed with the cellobiose molecule. In order to clarify the reason for this change in conformational preference, a subset of the cellobiose-water complexes presented in the preceding work are presented here in more detail. The relative energies of the cellobiose-water complexes are partitioned into cellobiose conformer energy, cellobiose-water interaction energy, and the stress energies required to distort the cellobiose and water molecules to the geometry of the complex. Water placement in a position bridging between the two sugar rings tends to destabilize the anti forms of cellobiose and stabilize the syn conformers. The source of the syn form stability in solution is found to lie in the characteristically stronger cellobiose-water interaction energies for the syn conformers, indicating the onset of a cooperativity in hydrogen bonding in the syn form of cellobiose due to the addition of the water molecules.