NMR investigation of a tetrasaccharide using residual dipolar couplings indilute 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 re sidual dipolar couplings in two lyotropic liquid crystalline media: (i) pre pared from mixtures of dimyristoyl phosphatidylcholine and dihexanoyl phosp hatidylcholine in water, and (ii) a ternary system consisting of cetylpyrid inium chloride/n-hexanol/brine (200 mM NaCl in water). Computer simulations , both Monte Carlo and molecular dynamics, were performed where different f orce fields were employed. The molecular structures generated were used in the analysis of the experimental dipolar C-H couplings. This analysis indic ated that in addition to different orientational tensors, a small conformat ional difference may be present for the oligosaccharide in the two media. T his interpretation emerged from a procedure where the effect of rotation ar ound one central glycosidic torsion angle on dipolar couplings was investig ated. Thus, the approach based on measurement of dipolar couplings is a sen sitive tool for investigation of molecular conformation.