Density Functional Calculations on Disaccharide Mimics: Studies of Molecular Geometries and Trans-O-glycosidic 3JCOCH and 3JCOCC Spin-Couplings

Abstract

Density functional theory (DFT) using the B3LYP functional and the split-valence 6-31G* basis set has been used to investigate the structures and conformations of four β-[1→4]-linked disaccharide mimics, 1−4. Systematic functional group substitution at sites near the glycosidic linkage was used to evaluate the effects of sterics and interresidue hydrogen bonding on the preferred glycosidic torsion angles φ and ψ. Using DFT-optimized geometries, the same hybrid functional, and a specially designed basis set, vicinal NMR scalar coupling constants involving carbon (3JCOCH, 3JCOCC) across the glycosidic linkages of 1−4 were calculated as a function of the φ and ψ torsion angles, and the resulting torsional dependencies were compared to recently reported experimental Karplus curves for these coupling pathways (Bose, B.; et al. J. Am. Chem. Soc. 1998, 120, 11158−11173). The new computational data are in excellent agreement with experimental results and confirm the general shape of the experimental curves. For 3...