MODIFICATION OF NATURAL POLYMERS BY THERMO-MECHANICAL PROCESSING
Location: Plant Polymer Research
Title: DENSITY FUNCTIONAL STUDY OF CELLOBIOSE HYDRATES
Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: August 11, 2004
Publication Date: October 18, 2004
Citation: Bosma, W.B., Appell, M.D., Willett, J.L., Momany, F.A. 2004. Density functional study of cellobiose hydrates [abstract]. American Chemical Society. p.189.
Previous studies of the disaccharide, cellobiose, at the B3LYP/6-311++G** level of theory led to a result in which the energetically favored conformations were those in which the glycosidic dihedral angles were in the range of (phi,psi) less than (180 degrees C, 0 degrees C). These conformers, called the "flipped" forms were less than 2.5 kcal/mol lower in energy than the "normal" form or those conformations experimentally observed in solution. To understand this result we have investigated the effect of adding water molecules around cellobiose, optimizing these configurations at the above DFT level of theory, and comparing the energy differences between the best "flipped" and best "normal" conformations. The resulting mono-, di-, tri-, and tetra-hydrate structures were found to converge on a energy difference that suggests that the synergetic hydrogen bonding network found for the "flipped" forms is gradually disrupted as more water molecules are added around the cellobiose molecule, allowing the "normal" conformation to become more favored energetically, relative to the "flipped" form. The energy differences with the "flipped" conformation favored moved from 2.5 kcal/mol with no water molecules, to less than 1.5 kcal/mol for one water, less than 1.0 kcal/mol for two waters, less than 0.6 kcal/mol for three water molecules, and finally to a value near zero for four water molecules. Clearly the preference for the "flipped" form is lost as the hydration level increases and we expect the "normal" form to be somewhat preferred in water.