Submitted to: Structural Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 20, 2007
Publication Date: November 1, 2007
Citation: Momany, F.A., Schnupf, U., Willett, J.L., Bosma, W. 2007. DFT study of alpha-maltose: influence of hydroxyl orientations on the glycosidic bond. Structural Chemistry. 18(5):611-632. Interpretive Summary: Starch is one of the most important biological molecules known, in addition to being one of the largest renewable crops on the planet. The three-dimensional structure of starch granules has to date defied description at the microscopic level. In particular, an understanding of the electronic structural and energetic features of maltose is advantageous, since maltose is the basic disaccharide component of starch polymers, containing one glycosidic bond between two glucose residues. In this paper results are presented on the electronic structure of 63 maltose conformations using cutting edge computational methods. From the use of analysis tools, experimental structural observations obtained by other researchers can be directly related to the computed results. This work has allowed better understanding of the flexibility and structural organization of amylose components in starch molecules, and will lead to more efficient design methods for chemical modifications of starch, resulting in biodegradable polymers with physical and structural properties useful for commercial applications.
Technical Abstract: The result of DFT geometry optimization of 68 unique alpha-maltose conformers at the B3LYP/6-311++G** level of theory is described. Particular attention is paid to the hydroxyl group rotational positions and their influence on the glycosidic bond dihedral angles. The orientation of lone pair electrons across the bridging hydrogen bonds are implicated in directing the glycosidic dihedral angles with for example, conformers gg-gg and gt-gt, having different minimum energy conformations for the clockwise (c) and the reverse clockwise (r) forms. Conformers tg-gg, gg-tg, tg-tg, gt-gg and gg-gt were studied, to understand the intermediate glycosidic bond conformations. The conformation, tg-gg-c, was found to be the lowest energy structure. When the hydroxyl groups on each glucose residue were made to point in opposite directions, i.e. c/r and r/c, the optimized structures were found to have high relative energies. Several optimized 'kink' structures were found around (phi, psi) less than (-40 deg C, -40 deg C), the lowest relative energy conformation being less than 3 kcal/mol. "Kink" conformations are observed in crystalline CA-10 and CA-14mers. Band-flip conformations, also observed in X-ray structures of CA-26 fragments, were studied with the lowest energy alpha-maltose conformations less than 4.0 kcal/mol above the global energy minimum. Several trends in geometry resulting from hydroxyl rotamer directions are described.