1H-NMR-Untersuchungen an Sialinsäuren, V1N-Acetyl-α-D-Neuraminsäure: Chemische Verschiebungen und Kopplungskonstanten; Konfiguration und Konformation in Lösung

A two-dimensional 1H-n.m.r. (500 MHz) and 13C-n.m.r. (125 MHz) study of N-linked glycopeptides derived from calf fetuin

The oligosaccharide part of an N-linked triantennary glycopeptide from calf fetuin with fourteen carbohydrate residues and its smaller derivatives obtained by successive enzymic cleavage of the terminal residues were investigated using 2D 1H-n.m.r. (500 MHz) and 13C-n.m.r. (125 MHz) spectroscopy. Assignments have been made of the resonances of almost all the protons of the constituent carbohydrate residues in these glycopeptides. A comparison of the 1H chemical shifts and coupling constants, as determined from the cross-peaks, has shown the dependence of these parameters on the interactions of spatially related neighbouring carbohydrates. Small conformational changes take place upon elongation of the oligosaccharide side-chains.

Elucidation of glycolipid structure by proton nuclear magnetic resonance spectroscopy

The primary structure of the oligosaccharide moiety of a glycosphingolipid can be elucidated by employing high-field proton nuclear magnetic resonance (NMR) spectroscopy. Information with respect to the composition and configuration of its sugar residues, and the sequence and linkage sites of the oligosaccharide chain can be obtained by employing a variety of one- and two-dimensional techniques. The latter include both scalar and dipolar correlated two-dimensional NMR spectroscopy. These techniques are also useful in establishing the solution conformation (secondary structure) of the oligosaccharide moiety. Examples in utilizing these techniques in elucidating the primary and secondary structures of glycolipids are presented.

The solution structure of mucous glycoproteins: proton NMR studies of native and modified ovine submaxillary mucin

The solution structure of native and systematically modified ovine submaxillary mucin (OSM) has been probed by proton NMR spectroscopic methods. Most of the resonances in the spectra have been tentatively assigned to the peptide and O-linked disaccharide, alpha-N-acetylneuraminic acid 2----6 alpha-N-acetylgalactosamine protons. On the basis of the observed chemical shifts, spectral resolution, and behavior of the exchangeable protons it is concluded the mucin possesses internal segmental flexibility and exists in solution as a random coil peptide. No long-lived interresidue peptide or carbohydrate hydrogen bonds were detected. The removal of (i) the C8 and C9 carbons of the sialic acid residue, (ii) the entire sialic acid residue, and (iii) the complete disaccharide side chain resulted in no significant changes in peptide core conformation. A limited set of proton spin coupling constants and nuclear Overhauser enhancements has been obtained for the threonine glycopeptide side chains in native and modified mucin. The results are consistent with the previously reported conformations for the (1----6) linkage in oligosaccharides and the threonyl glycosidic linkage in glycopeptides. The OSM disaccharide may exist as a extended linear structure with rotational freedom about the GalNAc C5-C6 bond, while the threonine glycosidic linkage appears to be sterically constrained, although multiple conformations about the threonine C beta-O gamma bond may be allowed. The small chemical shift perturbations detected in the glycosylated threonine methyl protons and the GalNAc carbons upon removal of the terminal sialic acid residue are consistent with this model.