Development of NMR methods for conformational analysis of ¹³C-enriched oligosaccharides

Abstract

The conformation and dynamics of sialyl Lewisx and related oligosaccharides were investigated using high resolution nuclear magnetic resonance measurements, and molecular dynamics calculations. In order to increase the number of structural parameters for inclusion in the molecular modelling simulations, the oligosacchaiide sialyl Lewisx was chemo-enzymatically synthesised to a high degree of carbon-13 enrichment (<99%). The incorporation of labelling allows editing of standard homonuclear NMR experiments by the 13C chemical shift, thus overcoming the spectral overlap which plagues carbohydrate NMR. Three dimensional heteronuclear NOESY/ROESY-HSQC experiments have allowed the unambiguous assignment and quantitation of NOEs/ROEs in a number of oligosaccharides including evidence that the anti-conformer is populated in aqueous solution for the Gal 1- 4GlcNAc linkage in sialyl 2,3-N-acetyllactosamine trisaccharide. Additional structural information was gained from the measurement of trans-glycosidic three bond carbon- carbon coupling constants. A Karplus relationship was derived for the C-O-C-C fragment allowing the back-calculation of 3Jcocc values from molecular dynamics simulation for comparison to experimental data. Additional distance restraints were derived from NOEs to exchangeable protons in low temperature studies using a mixed solvent system of H2O/acetone-d6. The increase in the number of distance restraints, have allowed the use time-averaged molecular dynamics simulations, and from these it is shown that NOEs to certain hydroxyls in sialyl Lewisx can only be explained if these hydroxyls are constrained within hydrogen bonds.