Submitted to: Carbohydrate Research
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/5/2005
Publication Date: 1/18/2006
Citation: Momany, F.A., Appell, M.D., Willett, J.L., Schnupf, U., Bosma, W.B. 2006. DFT study of A- and B-D-galactopyranose at the B3lYP/6-311++G** level of theory. Carbohydrate Research. p.525-537. Interpretive Summary: This work is a continuation of high level theoretical studies of a series of epimers of glucose. In this paper we present electronic structural characterization of D-galactopyranose, and epimer of D-glucose in which the four position hydroxyl group is moved from the equatorial to the axial configuration. These studies are carried out using powerful computer methods employed in our laboratory. With these computational tools, we can relate previous information from structural observations obtained by other researchers to details on the basic structure of galactose. This work has allowed us to better understand the flexibility and structural organization of the building blocks of naturally occurring carbohydrates. These studies will lead to more efficient design methods for chemical modifications of starch or polymer blends integrated with starch that will ultimately result in new biodegradable polymers with physical and structural properties useful for numerous commercial applications.
Technical Abstract: Forty-one conformations of alpha- and Beta-D-galactopyranose, the C-4 substituted epimer of glucopyranose, were geometry optimized using the B3LYP density functional and the 6-311++G** basis set. Full geometry optimization was performed on different ring geometries and different hydroxymethyl rotamers (gg/gt/tg). Analytically derived Hessians were used to calculate zero point energy, enthalpy, and entropy. The lowest energy and free energy conformation found is the x-gg-4C1-c chair conformation, which is of lower electronic and free energy than the lowest energy x-D-glucopyranose conformer because of favorable hydrogen bonding interactions. The in vacuo calculations showed considerable (less than 2.2 kcal/mol) energetic preference for the x- over the B-anomer for galactopyranose in both the 4C1 and 1C4 chair conformations. Results are compared to glucopyranose and mannopyranose calculations in vacuo. Boat and skew-boat forms were found that remained stable upon gradient optimization, although many starting conformations moved to other boat forms upon optimization. As with glucopyranose and mannopyranose, the orientation and interaction of the hydroxyl groups make the most significant contributions to the conformation/energy relationship in vacuo.