Derivatization with 1-Pyrenyldiazomethane Enhances Ionization of Glycopeptides but Not Peptides in Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry

Glycoproteomics holds the promise of new advances in medical technology. However, mass spectrometry has limitations for the structural determination of glycosylated peptides because the hydrophilic nature of the oligosaccharide moiety in glycopeptides is disadvantageous for ionization, and glycopeptides ionize much less readily than nonglycosylated peptides. Therefore, conventional proteomics tools cannot detect altered glycosylation on proteins. Here, we describe an on-plate pyrene derivatization method using 1-pyrenyldiazomethane for highly sensitive matrix-assisted laser/desorption ionization-tandem mass spectrometry (MALDI-MSn) of glycopeptides in amounts of less than 100 fmol. This derivatization is unique, as the pyrene groups are easily released from glycopeptides during ionization when 2,5-dihydroxybenzoic acid is used as a matrix. As a result, most ions are observed as the underivatized form on the spectra. At the same time, pyrene derivatization dramatically reduces the ionization of peptides. Thus, for glycopeptides in a mixture of abundant peptides, we could obtain MS spectra in which the signals of glycopeptides were intense enough for subjection to MSn in order to determine the structures of both glycan and peptide. Finally, we show that the glycopeptides derived from as little as 1 ng of prostate specific antigen can be detected by this method.