NMR investigation of a tetrasaccharide using residual dipolar couplings in dilute liquid crystalline media:: Effect of the environment

The tetrasaccharide lacto-N-neotetraose, beta-D-Galp-(1-->4)-beta-D-GlcpNAc-(1-->3)-beta-D-Galp-(1-->4)-D-Glcp, was investigated by measurements of residual dipolar couplings in two lyotropic liquid crystalline media: (i) prepared from mixtures of dimyristoyl phosphatidylcholine and dihexanoyl phosphatidylcholine in water, and (ii) a ternary system consisting of cetylpyridinium chloride/n-hexanol/brine (200 mM NaCl in water). Computer simulations, both Monte Carlo and molecular dynamics, were performed where different force fields were employed. The molecular structures generated were used in the analysis of the experimental dipolar C-H couplings. This analysis indicated that in addition to different orientational tensors, a small conformational difference may be present for the oligosaccharide in the two media. This interpretation emerged from a procedure where the effect of rotation around one central glycosidic torsion angle on dipolar couplings was investigated. Thus, the approach based on measurement of dipolar couplings is a sensitive tool for investigation of molecular conformation.