Direct N-glycan profiling in the presence of tryptic peptides on MALDI-TOF by controlled ion enhancement and suppression upon glycan-selective derivatization.

Even though the formidably laborious and time-consuming nature of oligosaccharide analysis limits certain attempts to analyze the glycosylation profile, the significant elucidation of carbohydrate modifications is largely dependent on it. Aiming to substantially improve the sample preparation procedure, a novel protocol allowing glycan-specific detection in the presence of other species, such as tryptic peptides, on MALDI-TOF was proposed and then evaluated. The new protocol is based on the concept that the desorption/ionization efficiency of glycans could be selectively and substantially enhanced while drastically suppressing the other ion species upon glycan-selective derivatization. A series of known and novel labeling reagents, all of which carry hydrazide functionality to allow glycan-specific derivatization, were prepared and evaluated in terms of their abilities to enhance the detection sensitivity of glycans, suppress ions of other contaminants (e.g., peptides), and detect acidic oligosaccharides. Several novel reagents that possess hydrophobic residue(s) together with quaternary ammonium/pyridinium or guanidino functionalities significantly enhanced the detection sensitivity of oligosaccharides. When enzymatically deglycosylated tryptic ovalbumin digest was directly derivatized by these reagents and subjected to MALDI-TOF analysis without any prior purification, we observed that a single type of analyte ion (labeled glycan) could suppress a large majority of peptide ions while allowing a low-femtomole level detection of oligosaccharides. The efficacy of this approach was further evaluated using several other model glycoproteins, including alpha(1)-acid glycoprotein that contains a variety of sialylated oligosaccharides.