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-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 ( 3 J COCH, 3 J COCC ) 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 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° 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.