Computational studies on carbohydrates: I. Density functional ab initio geometry optimization on maltose conformations

Ab initio geometry optimization was carried out on 10 selected conformations of maltose and two 2-methoxytetrahydropyran conformations using the density functional denoted B3LYP combined with two basis sets. The 6-31G∗ and 6-311++G∗∗ basis sets make up the B3LYP/6-31G∗ and B3LYP/6-311++G∗∗ procedures. Internal coordinates were fully relaxed, and structures were gradient optimized at both levels of theory. Ten conformations were studied at the B3LYP/6-31G∗ level, and five of these were continued with full gradient optimization at the B3LYP/6-311++G∗∗ level of theory. The details of the ab initio optimized geometries are presented here, with particular attention given to the positions of the atoms around the anomeric center and the effect of the particular anomer and hydrogen bonding pattern on the maltose ring structures and relative conformational energies. The size and complexity of the hydrogen-bonding network prevented a rigorous search of conformational space by ab initio calculations. However, using empirical force fields, low-energy conformers of maltose were found that were subsequently gradient optimized at the two ab initio levels of theory. Three classes of conformations were studied, as defined by the clockwise or counterclockwise direction of the hydroxyl groups, or a flipped conformer in which the ψ-dihedral is rotated by ∼180◦. Different combinations of ω side-chain rotations gave energy differences of more than 6 kcal/mol above the lowest energy structure found. The lowest energy structures bear remarkably close resemblance to the neutron and X-ray diffraction crystal structures. c © 2000 John Wiley & Sons, Inc.∗ J Comput Chem 21: 1204–1219, 2000