Submitted to: Carbohydrate Research
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/20/2005
Publication Date: 5/31/2005
Citation: Momany, F.A., Appell, M.D., Willett, J.L., Bosma, W.B. 2005. B3lyp/6-311++g** geometry optimization study of pentahydrates of alpha- and beta-d-glucopyranose. Carbohydrate Research. 340:1638-1655. Interpretive Summary: Starch and cellulose are two of the most important biological molecules known, in addition to being the largest renewable bio-materials on the planet. Our goal is to understand the electronic structural features of amylose, amylopectin, maltose, cellobiose, and the building blocks of these macromolecules, i.e. glucose, since these polymer fragments are the basic components of starch and cellulose polymers. In this paper we present high level electronic structure results on penta-hydrates of alph/Beta-D-glucose. 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 interactions between glucose and multiple water molecules. This work has allowed us to better understand the flexibility and structural organization of components of solvated carbohydrates, as well as more accurately define the role of solvated D-glucose residues in making up amylose and amylopectin in a starch granule. This work will lead to more efficient design methods for chemical modifications of starch or polymer blends with starch that will result in biodegradable polymers with physical and structural properties useful for numerous commercial applications.
Technical Abstract: Five water molecules were placed in 37 different configurations around alpha- and Beta-D-glucopyranose in the gt, gg, and tg conformational states, and the glucose-water complexes were geometry optimized using density functionals at the B3LYP/6-311++G** level of theory. The five water molecules were organized in space and energy minimized using an empirical potential, AMB02C, and then further geometry optimized using DFT algorithms to minimum energy positions. Electronic energy, zero point vibrational energy, enthalpy, entropy, stress energy on glucose and the water cluster, hydrogen bond energy, and relative free energy were obtained for each configuration using thermodynamic procedures and an analytical Hessian program. The lowest energy complex was that of a clustering of water molecules around the 1- and 6-hydroxyl positions of the Beta-gt anomer. Configurations in which the water molecules created a favorable network completely around and under glucose were found to have low energy for both alpha- and Beta-anomers. Calculation of the alpha/Beta anomeric ratio using the zero point corrected energy gave, less than 32/68%, highly favoring the Beta-anomer in modest agreement with the experimental less than 36/64% value. The glucose pentahydrate is more Beta anomer preferred than found in our previous monohydrate study were an alpha/Beta ratio of less than 50/50% was found. An approximate hydroxymethyl population was obtained by noting average relative energies among the three conformational states, gt, gg, tg. The gg and gt states were favored over tg and in the Beta-anomer the gt conformation was favored, while in the alpha-anomer the gg state was favored, with the tg conformations of higher energy making little or no contribution to the gg/gt ratio. Some geometry variances, found between glucose in vacuo and glucose after interaction with water molecules, are described and account for some observed C-5-C-6 bond length anomalies reported by us previously for the vacuum glucose structures.