Density functional calculations on disaccharide mimics: Studies of molecular geometries and trans-O-glycosidic (3)J(COCH) and (3)J(COCC) spin-couplings

Density functional theory (DFT) using the B3LYP functional and the split-va lence 6-31G* basis set has been used to investigate the structures and conf ormations 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 th e preferred glycosidic torsion angles phi and psi. Using DFT-optimized geom etries, the same hybrid functional, and a specially designed basis set, vic inal 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 th e phi and psi, torsion angles, and the resulting torsional dependencies wer e compared to recently reported experimental Karplus curves for these coupl ing pathways (Bose, B.; et al. J. Am. Chem. Sec. 1998, 120, 11158-11173). T he new computational data are in excellent agreement with experimental resu lts and confirm the general shape of the experimental curves. For (3)J(COCH ), however, small discrepancies were observed at the extreme dihedral angle s, suggesting some deficiencies in the theory and/or experimental data. For (3)J(COCC), the new computed couplings confirm the existence of terminal e lectronegative substituent effects on coupling magnitude, and computed coup lings in the 0-100 degrees range of dihedral angles lead to an improved Kar plus curve for the interpretation of (3)J(COCC) values across the O-glycosi dic linkages of oligosaccharides.