Mass spectrometry and 13C nuclear magnetic resonance spectroscopy of compounds modeling the glycopeptide linkage of glycoproteins

Abstract

The properties of several compounds useful as models for three-dimensional conformational studies and the investigation of the chemical degradation of glycopeptide linkages both of the N- and O-glycosidic type are described. Using the method of differential chemical shift in H 2O and D 2O as solvents, the carbon NMR spectrum of N-acetylglucosaminylasparagine, 1- N-acetyl-β- d-glucopyranosylamine, and 1- N-acetyl-2-acetamido-β- d-glucopyranosylamine has been assigned. Electron impact mass spectra of the peracetylated derivatives of the latter two compounds show a peak apparently unique to glycopyranosylamides at m e = 269 , no analog of which is observed in the mass spectra of other peracetylated sugars. As models of the α- O-glycosidic linkage, fully assigned carbon NMR spectra of α-methyl- N-acetylgalactosamine (GalNAc), α-methyl-3- O-methyl GalNAc,and -GlcNAc as well as the disaccharide Glc-β-1 → 3 GalNAc are reported. Because certain anomalies in the chemical shifts and 1 J CH observed in the disaccharide and in O-glycosylated glycoproteins are not observed in the simple model compounds, they may result from conformational interactions in the glycopeptides.