STRUCTURAL AND CONFORMATIONAL-ANALYSIS OF SIALYLOLIGOSACCHARIDES USING C-13 NUCLEAR MAGNETIC-RESONANCE SPECTROSCOPY

The analysis of the carbon-13 chemical shift data of NeuAc.alpha.(2 .fwdarw. 3)Gal.beta.(1 .fwdarw. 4)Glc and NeuAc.alpha.(2 .fwdarw. 3)Gal.beta.-(1 .fwdarw. 4)GlcNAc and their respective NeuAc.alpha.(2 .fwdarw. 6) isomers established distinct and different conformations of the sialic acid residue, depending on the type of anomeric linkage [.alpha.(2 .fwdarw. 3) vs. .alpha.(2 .fwdarw. 6)]. Interactions between the NeuAc residue and the Glc or GlcNAc residue are particularly strong in the case of the .alpha.(2 .fwdarw. 6) isomers. Similar effects are observed for the larger oligosaccharides [II3(NeuAc)2Lac and IV6NeuAcLcOse4] and even in intact glycoproteins and polysaccharides. It is proposed that the NeuAc.alpha.(2 .fwdarw. 3) isomers assume an extended conformation with the sialic residue at the end (terminal) of the oligosaccharide chain or branch. The NeuAc.alpha.(2 .fwdarw. 6) isomers are assumed to be folded back toward the inner core sugar residues.