Characterizing sialic acid variants at the glycopeptide level.

Beam-type collision-induced dissociation (CID) data of intact glycopeptides isolated from mouse liver tissue are presented to illustrate characteristic fragmentation of differentially sialylated glycopeptides. Eight glycoforms of an O-linked glycopeptide from Nucleobindin-1 are distinguished on the basis of the precursor masses and characteristic oxonium ions. We report that all sialic acid variants are prone to neutral loss from the charge reduced species in electron-transfer dissociation (ETD) fragmentation. We show how changes in sialic acid composition affect reverse phase chromatographic retention times: sialic acid addition increases glycopeptide retention times significantly; replacing the N-acetylneuraminic acid with the N-glycolyl variant leads to slightly reduced retention times, while O-acetylated sialic acid-containing glycoforms are retained longer. We then demonstrate how MS-Filter in Protein Prospector can use these diagnostic oxonium ions to find glycopeptides, by showing that a wealth of different glycopeptides can be found in a published phosphopeptide data set.