Angular dependence of vicinal carbon-proton coupling constants for conformational studies of the hydroxymethyl group in carbohydrates

A theoretical study is presented of the dependence on the hydroxymethyl group conformation of vicinal carbon-proton coupling constant 3JC,H in a series of 16 hexopyranoses. Calculated 3JC,H values for both anomers of d-glucopyranose (1), d-mannopyranose (2), d-allopyranose (3), d-altropyranose (4), d-galactopyranose (5), d-talopyranose (6), d-gulopyranose (7), and d-idopyranose (8) are based on the FTP formulation in the semi-empirical approximation of INDO. The dependence of the coupling constants on the dihedral angle ωC between the coupling carbon atom C-4 and protons H-6 is represented by a trigonometric function of the form 3JC,H = 5.8cos2ωC − 1.6cosωC + 0.28sin2ωC − 0.02sinωC + 0.52. It was found that the configuration at the anomeric and C-4 carbon atoms does not show any significant influence on 3JC,H values. Agreement of calculated and experimental values available for mono- and oligo-saccharides is satisfactory. Based on these results, it is concluded that proposed equation for 3JC,H values can be used as a tool for estimation of the conformational properties of the hydroxymethyl group in monosaccharides and of (1 → 6)- linked oligosaccharides in solution.