Tandem Mass Spectrometry Analysis of <i>O</i>-Glycopeptides

In recent years the significance of O-glycans has become known, and there has been a demand for the analysis of O-glycopeptides. Analysis of O-glycopeptides has not progressed as much as that of N-glycopeptides, because, in conventional tandem mass spectrometry (MS/MS) measurements, O-glycans are easily detached from the peptides. This results in the loss of information on the glycosylation site, making it difficult to identify the glycopeptide containing the glycosylation site. Recently, MS/MS methods such as electron transfer dissociation (ETD) that fragment only the backbone of the peptide chain without releasing the glycans have been developed. Here, I summarize recent progress in the analysis of O-glycopeptides and discuss the prospects for their analysis.