Location: Plant Polymer Research
Title: DFT STUDY OF CARBOHYDRATES: A COMPARISON BETWEEN THE EPIMERS OF GLUCOSE Authors
Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: May 17, 2006
Publication Date: June 15, 2006
Citation: Schnupf, U., Willett, J.L., Momany, F.A. 2006. DFT study of carbohydrates: A comparison between the epimers of glucose [abstract]. Midwest Chemistry Conference. n.10. Technical Abstract: DFT calculations have been carried out at the B3LYP/6-311++G** level of theory on the low energy structures of glucose and its epimers allose, mannose, galactose, altrose, gulose, talose and idose. The full geometry optimizations were performed on different ring geometries (chairs, boat and skew-boat) and different hydroymethyl rotamers (gg/gt/tg). Furthermore, the exocyclic hydroxyl groups were considered in either the all clockwise or "c" form, or the all reverse-clockwise or "r" form. Analytical Hessians were used to calculate zero point energy, enthalpy, entropy, and relative Gibbs Free energies are reported at the harmonic level of theory. Surprisingly, it was found for the first time at the given level of theory, that two of the epimers of glucose, galactose and talose, are energetically lower than glucose. For all the epimers the preferred low energy conformation is the 4C1 chair with the lowest belonging to alpha-D-talose (2,4 axial hydroxyl groups) because of favorable hydrogen-bonding interactions. The in vacuo calculations showed energetic preference for the alpha-anomer over the Beta-anomer for most of the epimers in their 4C1 conformation. In contrast, for the 1C4 conformation half the epimers prefer the Beta-anomer. Boat and skew-boat conformations have been found of modestly higher energy (Delta-E less than 5 kcal/mol and higher) relative to the lowest energy structures. A comparison of the relative energies clearly indicates that the orientation of the hydroxyl groups make the most significant contributions to the conformation-energy relationship in vacuo. In addition to the vacuo calculations solvation studies have been carried out using a continuum-solvation model (COSMO). For a selected group of vacuo and solvated epimer geometries alpha-Beta-anomeric ratios have been calculated from the relative free energies. A comparison of the difference in alpha-Beta-anomeric ratios between the vacuum structures and solvated (COSMO) structures will be given.