Structural identification of N-glycan isomers using logically derived sequence tandem mass spectrometry.

N-linked glycosylation is one of the most important protein post-translational modifications. Despite the importance of N-glycans, the structural determination of N-glycan isomers remains challenging. Here we develop a mass spectrometry method, logically derived sequence tandem mass spectrometry (LODES/MS), to determine the structures of N-glycan isomers that cannot be determined using conventional mass spectrometry. In LODES/MS, the sequences of successive collision-induced dissociation are derived from carbohydrate dissociation mechanisms and apply to N-glycans in an ion trap for structural determination. We validate LODES/MS using synthesized N-glycans and subsequently applied this method to N-glycans extracted from soybean, ovalbumin, and IgY. Our method does not require permethylation, reduction, and labeling of N-glycans, or the mass spectrum databases of oligosaccharides and N-glycan standards. Moreover, it can be applied to all types of N-glycans (high-mannose, hybrid, and complex), as well as the N-glycans degraded from larger N-glycans by any enzyme or acid hydrolysis.