Parameterization of a prediction equation for NMR-²J_CH couplings of α-D-galactose based on the conformational analysis

Abstract

Two-bond ¹³C-¹H NMR spin-spin coupling constants can be described by the rotational angles of adjacent hydroxyl groups attached to pyranosyl ring, and those values can be used for structural constraints in the conformational determination of sugar chain in chemical and biochemical environment. In this study, to reproduce the experimental NMR coupling values and to predict sugar conformations in aqueous solution, two approaches have been examined. One is an exhaustive conformational analysis by using all conformers of α-D-galactose in aqueous solution. Energy and NMR coupling constant calculations for each conformation were performed by using high-level ab initio MO calculations with solvent effects. Although some conformers that comparatively reproduced the experimental ²J_CH values were found, these were not predominant conformers in the ab initio calculations. Another approach is a combination of MD calculations of sugar ensemble in experimental conditions of NMR measurement and new equations of NMR-²J_CH coupling values like as Karplus ³J_HH equation. Averaged NMR-²J_CH values applied the equations to the snapshot structures can reproduce the experimental slightly better than the former approach.