Comparison of NMR and molecular modeling results for a rigid and a flexible oligosaccharide.

Three-bond heteronuclear coupling constants (3JCH) are extremely useful in describing flexible models for oligosaccharides. We show that antiphase methods for measuring 3JCH in oligosaccharides have limited reliability but that the coupling constants can be reliably measured in natural abundance by quantitative J-correlation methods. Interpretation of 3JCH data for a pentasaccharide (lacto-N-fuco-pentaose 2) from human milk are consistent with a rigid model for the Lewis(a) trisaccharide epitope but for an antigenic tetrasaccharide fragment from the cell wall polysaccharide of viridans streptococci, 3JCH data imply a considerably more flexible model. Nuclear Overhauser effect (NOE) data are reported for a heptasaccharide repeating unit isolated from the cell wall polysaccharide of Streptococcus gordonii 38. The results for a tetrasaccharide fragment are similar to data reported for the same fragment in the cell wall polysaccharide from S.mitis J22. This result implies a similar conformation for the tetrasaccharide fragment in the polysaccharide and in the heptasaccharide and also implies that anisotropy of motion is not significant in the interpretation of the nuclear Overhauser effects in the polysaccharide. Interpretation of the NOE results for the tetrasaccharide fragment, like the 3JCH data, implies a flexible model with three conformations in fast exchange. The results of the two experimental techniques are combined with molecular modeling results including molecular dynamics simulation to provide a clear delineation between flexible and rigid oligosaccharide epitopes. The blood group Lewis(a) trisaccharide antigenic determinant is highly restricted in its motions by steric interactions while the antigenic tetrasaccharide fragment of the S.gordonii 38 heptasaccharide is considerably more mobile. We propose that some branched oligosaccharides are relatively rigid and some are flexible depending on subtle details of the linkages.