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

Abstract

Optimized geometries and total energies of some conformers of alpha- and beta-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 alpha- and beta-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 alpha 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 psi and phi of the glycosidic linkage are considered. In each calculation, at each increment of psi and phi, using a step of 30 degrees from 0 to 330 degrees, 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*.

Conformation, dynamics, solvation and relative stabilities of selected β-hexopyranoses in water: a molecular dynamics study with the gromos 45A4 force field

“…162,163,338 For Man the available quantum-mechanical results 255,337,342 also suggest a counterclockwise network in the lowest-energy conformer, but along with a H 6 !O 4 hydrogen-bond. The same counterclockwise network was also found in previous quantummechanical calculations on Gal or its O 1 -methylated derivative in vacuum, 104,162,163,255,337,340,[343][344][345][346] although most of these calculations (see, however 104,343 ) as well as IR ion-dip experiments on phenyl-b-D D-galactopyranoside in the gas-phase 162,163,344 suggested the presence of a H 6 !O 5 hydrogen-bond (as in Glc) rather than a H 6 !O 4 hydrogen-bond (present simulation) in the lowest-energy conformer. Finally, for Tal, the only quantum-mechanical study available to our knowledge also supports the presence of a H 6 !O 5 hydrogenbond.…”

Infrared diagnostic of the conformational change of methylated β-glucose bound to Li+ upon ligand addition

“…Furthermore, the lowest energy structure of the Na(β-Glc) + system has the β-glucose in a boat conformation and acting as a tridentate ligand, very similar to the lowest energy β-Glc-B-356 structure found for Li(β-Me-Glc) + . The lowest energy structures calculated by Armentrout and coworkers were consistent with the previous results obtained by Kentäma and coworkers for the arabinose and xylose systems, 26 with Barrows et al 25 for the glucose systems, and Rahal-Sekkal et al 30 for galactose saccharide. However, these results contrast with earlier results from Wesdemiotis and coworkers 31 on sodiated monosaccharide and disaccharide complexes.…”

Quantum Chemical Determination of Young's Modulus of Lignin. Calculations on a β−O-4‘ Model Compound

“…In a study on a mono-and disaccharide, it was reported that AM1 and HF/6-31G* resulted in similar structural results. 26 Among the methods used in the current Article, B3LYP/6-31G* is the most accurate for the determination of energies, followed by AM1, with much larger errors from the Hartree-Fock calculations. 25 The current study is not intended to be a systematic examination of computational methods, or a documentation of differences between theory and experiment.…”