Epimeric and anomeric relationship of octyl- α - d -gluco/galactosides: Insight from density functional theory and atom in molecules studies

Density functional theory calculations on three conformers, gt (gauche-trans), gg (gauche-gauche) and tg (trans-gauche) of n-octyl-α-d-glucoside (C8O-α-Glc) and n-octyl-α-d-galactoside (C8O-α-Gal) were performed for geometry optimization at the B3LYP/6-311++G∗∗ level. Both of these molecules are epimers differing only in the orientation of the hydroxyl group at the C4 position. We investigated electronically the effect of direction (axial/equatorial) of the hydroxyl group at C4 position and the orientation of hydroxymethyl rotamer on C6 position. The structure parameters of XH⋯Y intramolecular hydrogen bonds (HB) were analyzed in gas and solvent phases while the nature of these bonds was characterized using the Atoms in Molecules (AIM) approach. C8O-α-Glc (in gg and tg conformers) has two intramolecular hydrogen bonds and is more stable than the corresponding gt conformer with only one intramolecular hydrogen bond. The ellipticity of the conventional hydrogen bond (HO4⋯O6 and HO4⋯O3) is found to be much lower than the unconventional (HC7⋯O2). In addition, the number of intramolecular hydrogen bonds (conventional and unconventional) stabilizes the resulting self-assemblies from epimers and anomers of these glycolipids and supports the previous experimental observations. The C8O-β-Gal has two conventional hydrogen bonds and has the highest liquid crystal clearing transition temperature, whereas C8O-α-Gal and C8O-β-Glc with only one conventional hydrogen bond have the least clearing temperature.