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: 2. Energy contributions to relative stabilities of cellobiose (H2O)1-4 complexes. Journal of Molecular Structure (Theochem). 776:21-31. Interpretive Summary: In a continuation of the studies described in paper #1, the energies involved in the cellobiose-water interactions were carefully analyzed, by performing separate calculations on the cellobiose and water molecules without letting them relax back to their normal structures. This technique allows the computational model to isolate the reason that one cellobiose conformer is more stable than the other in solution, while the relative stabilities of the structures reverse upon dissolving cellobiose in water. It was found that very strong favorable cellobiose-water interactions occur when a water molecule is placed between the two sugar rings in the solution-phase, but the vacuum favored cellobiose conformer does not realize these same attractive forces, and thus becomes less stable when the cellobiose molecule is placed in water. These results bear greatly on studies in which cellulose is dissolved to obtain material useful for ethanol production.
Technical Abstract: 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 sources 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.