Determination of O-glycosylation heterogeneity using a mass-spectrometric method retaining sugar modifications.

A mass-spectrometric method for a de novo determination of O-glycosylation heterogeneity was developed. We used a mild fragmentation technique, electron capture dissociation (ECD), which enables the determination of glycosylation sites as well as peptide sequencing. To demonstrate the correct identification of glycopeptides, we prepared a series of glycopeptides with the same peptide sequence and 6 different glycan modifications. ECD spectra were obtained at various electron energies, and were analyzed with the Mascot database-search engine. The obtained candidate glycopeptides were further validated by confirming the spectral overlap of ECD fragment peaks with the theoretical peaks. The results indicate that all glycopeptides were unambiguously identified, including glycosylation sites by combining ECD results with different electron energies for each glycopeptide.