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

Three-bond heteronuclear coupling constants (³J_CH) are extremelyuseful in describing flexible models for oligosaccharides. Weshow that antiphase methods for measuring ³J_CH in oligosaccharideshave limited reliability but that the coupling constants canbe reliably measured in natural abundance by quantitative J-correlationmethods. Interpretation of ³J_CH data for a pentasaccharide (lacto-N-fuco-pentaose2) from human milk are consistent with a rigid model for theLewis^a trisaccharide epitope but for an antigenic tetrasaccharidefragment from the cell wall polysaccharide of viridans streptococci,³J_CH data imply a considerably more flexible model. NuclearOverhauser effect (NOE) data are reported for a heptasacchariderepeating unit isolated from the cell wall polysaccharide ofStreptococcus gordonii 38. The results for a tetrasaccharidefragment are similar to data reported for the same fragmentin the cell wall polysaccharide from S.mitis 322. This resultimplies a similar conformation for the tetrasaccharide fragmentin the polysaccharide and in the heptasaccharide and also impliesthat anisotropy of motion is not significant in the interpretationof the nuclear Overhauser effects in the polysaccharide. Interpretationof the NOE results for the tetrasaccharide fragment, like the³J_CH data, implies a flexible model with three conformationsin fast exchange. The results of the two experimental techniquesare combined with molecular modeling results including moleculardynamics simulation to provide a clear delineation between flexibleand rigid oligosaccharide epitopes. The blood group Lewis^a trisaccharideantigenic determinant is highly restricted in its motions bysteric interactions while the antigenic tetrasaccharide fragmentof the S.gordonii 38 heptasaccharide is considerably more mobile.We propose that some branched oligosaccharides are relativelyrigid and some are flexible depending on subtle details of thelinkages. oligosaccharide conformation molecular dynamics