Conformational analysis

Density Functional Calculations on Disaccharide Mimics: Studies of Molecular Geometries and TransOglycosidic 3JCOCH and 3JCOCC SpinCouplingsCloran F, Carmichael I, Serianni AS; J Am Chem Soc 121 (1999) 98439851 AbstractDensity functional theory (DFT) using the B3LYP functional and the splitvalence 631G* basis set has been used to investigate the structures and conformations of four b[14]linked disaccharide mimics, 14. 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 DFToptimized 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 14 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. Soc. 1998, 120, 1115811173). The new computational data are in excellent agreement with experimental results and confirm the general shape of the experimental curves. For 3JCOCH, however, small discrepancies were observed at the extreme dihedral angles, suggesting some deficiencies in the theory and/or experimental data. For 3JCOCC, the new computed couplings confirm the existence of terminal electronegative substituent effects on coupling magnitude, and computed couplings in the 0100 range of dihedral angles lead to an improved Karplus curve for the interpretation of 3JCOCC values across the Oglycosidic linkages of oligosaccharides. Equation^{3}J=4.96*cos^{2}(θ)0.52*cos(θ)+0.18
