Density functional calculations on disaccharide mimics:: Studies of molecular geometries and trans-O-glycosidic 3JCOCH and 3JCOCC spin-couplings

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 beta-[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 phi and psi. Using DFT-optimized geometries, the same hybrid functional, and a specially designed basis set, vicinal NMR scalar coupling constants involving carbon ((3)J(COCH), (3)J(COCC)) across the glycosidic linkages of 1-4 were calculated as a function of the phi and psi, 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. Sec. 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)J(COCH), however, small discrepancies were observed at the extreme dihedral angles, suggesting some deficiencies in the theory and/or experimental data. For (3)J(COCC), the new computed couplings confirm the existence of terminal electronegative substituent effects on coupling magnitude, and computed couplings in the 0-100 degrees range of dihedral angles lead to an improved Karplus curve for the interpretation of (3)J(COCC) values across the O-glycosidic linkages of oligosaccharides.