Assignment of Saccharide Identities through Analysis of Oxonium Ion Fragmentation Profiles in LC–MS/MS of Glycopeptides

Protein glycosylation plays critical roles in the regulation of diverse biological processes, and determination of glycan structure–function relationships is important to better understand these events. However, characterization of glycan and glycopeptide structural isomers remains challenging and often relies on biosynthetic pathways being conserved. In glycoproteomic analysis with liquid chromatography–tandem mass spectrometry (LC–MS/MS) using collision-induced dissociation (CID), saccharide oxonium ions containing <i>N</i>-acetylhexosamine (HexNAc) residues are prominent. Through analysis of beam-type CID spectra and ion trap CID spectra of synthetic and natively derived N- and O-glycopeptides, we found that the fragmentation patterns of oxonium ions characteristically differ between glycopeptides terminally substituted with GalNAcα1-<i>O</i>-, GlcNAcβ1-<i>O</i>-, Galβ3GalNAcα1-<i>O</i>-, Galβ4GlcNAcβ-<i>O</i>-, and Galβ3GlcNAcβ-<i>O</i>- structures. The difference in the oxonium ion fragmentation profiles of such glycopeptides may thus be used to distinguish among these glycan structures and could be of importance in LC–MS/MS-based glycoproteomic studies