DFT study of α-maltose: influence of hydroxyl orientations on the glycosidic bond

Abstract

The result of DFT geometry optimization of 68 unique α-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 ( $$\phi_{\rm H}$$ , ψH) ∼(−40°, −40°), the lowest relative energy conformation being ∼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 α-maltose conformations ∼4.0 kcal/mol above the global energy minimum. Several trends in geometry resulting from hydroxyl rotamer directions are described.