Submitted to: Carbohydrate Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/16/2003
Publication Date: 1/30/2004
Citation: Appell, M., Strati, G., Willett, J.L., Momany, F.A. 2004. B3LYP/6-311++G** study of z-and B-D-glucopyranose and 1,5-anhydro-D-glucitol:4C1 and 1C4 chairs, 3,0 B and B 3,0 boats, and skew-boat conformations. Carbohydrate Research. n. 339. p. 537-551. Interpretive Summary: Starch and cellulose are two of the most important naturally occurring compounds known, in addition to being the largest renewable bio-materials on the planet. Unfortunately, the 3-dimensional structure of starch granules has to date defied precise description at the atomic and molecular level, while cellulose has become somewhat better understood. In particular, we require an understanding of the molecular electronic structural features of amylose, amylopectin, cellobiose, maltose, and a/B-D-glucose, since these molecules are the basic components of starch and cellulose polymers. In this paper we present high level electronic structure results on chair, boat, and skew-boat conformations of a/B-D-glucose. These studies are carried out using powerful computer methods employed in our laboratory. With these computational tools, we can directly relate previous information from structural observations such as obtained from atomic force microscopy to the basic flexibility and molecular structure of D-glucose. This work has allowed us to better understand the flexibility and structural organization of components of starch molecules, as well as to more accurately define the role of the D-glucose molecular structure in making up the starch granule. This work will lead to more efficient design methods for chemical modifications of starch that will result in biodegradable polymers with physical and structural properties useful for numerous commercial applications.
Technical Abstract: Geometry optimization, at the B3LYP/6-311++G** level of theory, was carried out on 4C1 and 1C4 chairs, 3,0B and B3,0 boats, and skew-boat conformations of a- and B-D-glucopyranose. Similar calculations on 1,5-anhydro-D-glucitol allowed examination of the effect of removal of the 1-hydroxy group on the energy preference of the hydroxymethyl rotamers. Stable minimum energy boat conformers of glucose were found, as were stable skew-boats, all having energies ranging from -4-15 kcal/mol above the global energy 4C1 chair conformation. The 1C4 chair electronic energies were -5-10 kcal/mol higher than the 4C1 chair, with the 1C4 a-anomers being lower in energy than the B-anomers. Zero point energy, enthalpy, entropy, and relative Gibbs free energies are reported at the harmonic level of theory. The a-anomer and 4C1 chair conformations were found to be -1 kcal/mol lower in electronic energy than the B-anomers. The hydroxymethyl gt conformation was of lowest electronic energy for both the a- and B-anomers. The glucose a/B anomer ration calculated from the relative free energies is 63/37%. From a numerical Hessian calculation, the tg conformations were found to be -0.4-0.7 kcal/mol higher in relative free energy than the gg or gt conformers. Transition state barriers to rotation about the C5-C6 bond were calculated for each glucose anomer with resulting barriers to rotation of -3.7-5.8 kcal/mol. No energy barrier was found for the path between the a-gt and a-gg B3,0 boat forms and the equivalent 4C1 chair conformations. The a-tg conformation has an energy minimum in the 1S3 twist form. Other boat and skew-boat forms are described. The B-anomer boats retained their starting conformations, with the exception of the B-tg-3,0B boat that moved to a skew form upon optimization.