Submitted to: Journal of Molecular Structure (Theochem)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/26/2006
Publication Date: 8/10/2006
Citation: Bosma, W., Appell, M.D., Willett, J.L., Momany, F.A. 2006. Stepwise hydration of cellobiose by DFT methods: 1. Conformational and structural changes brought about by the addition of one to four water molecules. Journal of Molecular Structure (Theochem). 776:1-19. Interpretive Summary: Cellulose is the most abundant naturally occurring polymer, forming the structural "backbone" of most plants. A better understanding of the structure of cellulose will allow scientists to develop ways to chemically modify this polymer in ways that make its structural and chemical properties more useful to society. To better understand the structure of cellulose in its natural environment, advanced theoretical methods were used to study cellobiose, the disaccharide sub-unit of cellulose. Cellobiose has been experimentally shown to undergo a major structural change from one conformer to another upon removing this carbohydrate from water. This conformational change was studied by modeling a cellobiose molecule with addition of one to four water molecules, with the aim of determining how many water molecules are required to bring about the desired conformational change. The studies showed that only two water molecules interacting with cellobiose will provide a 50/50 mixture of the two structures, and four water molecules force the cellobiose molecule to strongly prefer its solution-phase structure. Geometry changes that are associated with the addition of water molecules were analyzed to get a clearer picture of how the cellobiose molecule responds to dissolution in water.
Technical Abstract: 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 3 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.