Conformational analysis of sialyloligosaccharides.

The conformational properties of several sialyloligosaccharides present as terminal sequences in N- and O-linked carbohydrate groups of glycoproteins, have been analyzed based on the n.m.r. data of selected sialosides. The compounds examined include representatives of the alpha-D-NeuAc-(2----6)-beta-D-Gal-(1----4)-beta-D-GlcNAc, alpha-D-NeuAc-(2----3)-beta-D-Gal-(1----4)-beta-D-GlcNAc, alpha-D-NeuAc-(2----3)-beta-D-Gal-(1----3)-beta-D-GlcNAc, and alpha-D-NeuAc-(2----3)-beta-D-Gal-(1----3)-beta-D-GalNAc series. Two deuterated sialosides were prepared by enzymic sialylation of 6-deuterated galactose derivatives of methyl beta-D-galactopyranoside and lactoside. These were useful for the unambiguous establishment of the "gt" orientation of the flexible C-6 methylene unit of the galactose through 1H-1H coupling constants. Of all the (2----6) sialosides examined, only the deuterated di- and tri-saccharide afforded useful nuclear Overhauser enhancement data that could be used to evaluate the global minimum-energy conformations. Hard-sphere exoanomeric effect calculations estimated the glycosidic torsion angles for the global minimum-energy conformer of alpha-D-NeuAc-(2----6)-beta-D-Gal linkages to be -163/-132/61 degrees (theta, psi, and omega, respectively). However, the potential energy well surrounding this global minimum was very shallow and indicated a broad population distribution of conformers. These are illustrated by the isoenergy contour maps. The observation of n.O.e. between the H-3ax and H-6R of the galactose in two deuterated (2----6) sialosides, indeed supported the presence of one of the global minimum-energy conformers. The conformational analysis carried out for the di- and trisaccharide [alpha-D-NeuAc-(2----6)-beta-D-Gal-OMe and alpha-D-NeuAc-(2----6)-beta-D-Gal-(1----4)-beta-D-Glc-OMe respectively] was then extended to sialoside linkages of other tri- and penta-saccharides by comparison of their 1H- and 13C-n.m.r. chemical shifts. HSEA calculations for the (2----3) sialosides indicated the potential energy well containing the global minimum energy-conformer (theta, psi = -160 +/- 4, -11 +/- 2 degrees) was deeper than the one estimated for the (2----6) sialosides. The n.O.e. data are consistent with the distribution of the majority of conformers around the lowest-energy one in solution. CPK models highlighting the topographical differences between the lowest-energy conformations of alpha-(2----6) and alpha-(2----3) sialosides are presented.