Targeted, Site-specific quantitation of N- and O-glycopeptides using O-labeling and product ion based mass spectrometry.

The site-specific quantitation of N- and O-glycosylation is vital to understanding the function(s) of different glycans expressed at a given site of a protein under physiological and disease conditions. Most commonly used precursor ion intensity based quantification method is less accurate and other labeled methods are expensive and require enrichment of glycopeptides. Here, we used glycopeptide product (y and Y0) ions and O-labeling of C-terminal carboxyl group as a strategy to obtain quantitative information about fold-change and relative abundance of most of the glycoforms attached to the glycopeptides.