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Structures and energies of D-galactose and galabiose conformers as calculated by ab initio and semiempirical methods

✍ Scribed by Majda Rahal-Sekkal; Nezha Sekkal; Dirk C. Kleb; Paul Bleckmann

Publisher: John Wiley and Sons
Year: 2003
Tongue: English
Weight: 312 KB
Volume: 24
Category: Article
ISSN: 0192-8651
DOI: 10.1002/jcc.10223

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✦ Synopsis

Abstract

Optimized geometries and total energies of some conformers of α‐ and β‐D‐galactose have been calculated using the RHF/6‐31G* ab initio method. Vibrational frequencies were computed at the 6‐31G* level for the conformers that favor internal hydrogen bonding, in order to evaluate their enthalpies, entropies, Gibbs free energies, and then their structural stabilities. The semiempirical AM1, PM3, MNDO methods have also been performed on the conformers GG, GT, and TG of α‐ and β‐D‐galactose. In order to test the reliability of each semiempirical method, the obtained structures and energies from the AM1, PM3, and MNDO methods have been compared to those achieved using the RHF/6‐31G* ab initio method. The MNDO method has not been investigated further, because of the large deviation in the structural parameters compared with those obtained by the ab initio method for the galactose. The semiempirical method that has yielded the best results is AM1, and it has been chosen to perform structural and energy calculations on the galabiose molecule (the disaccharides constituted by two galactose units α 1,4 linked). The goal of such calculations is to draw the energy surface maps for this disaccharide. To realize each map, 144 different possible conformations resulting from the rotations of the two torsional angles ψ and ϕ of the glycosidic linkage are considered. In each calculation, at each increment of ψ and ϕ, using a step of 30° from 0 to 330°, the energy optimization is employed. In this article, we report also calculations concerning the galabiose molecule using different ab initio levels such as RHF/6‐31G*, RHF/6‐31G**, and B3Lyp/6‐31G*. © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 806–818, 2003

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