Investigation of a side reaction occurring during N-linked glycan labeling by cationic tags

Capillary electrophoresis-mass spectrometry was applied for the analysis of oligosaccharides and N-linked glycans with an attached charge label facilitating electrophoretic migration and electrospray ionization efficiency. Several different labeling strategies have been tested with different tags and tagging reactions including reductive amination and hydrazone formation. However, a formation of multiple labeled N-linked glycans was observed by CE-MS in a positive ion mode when positively charged labels such as aliphatic amines containing a quaternary ammonium group were attached to N-linked glycans by reductive amination. A reaction mechanism explaining a side reaction occurring during the labeling and the multiple product formation was proposed and confirmed by using isotopically labeled N-acetylglucosamine. Finally, it was confirmed that derivatization of sugars via a hydrazone formation can be a simpler method with a high reaction yield suitable for high sensitive CE-ESI/MS analyses of N-linked glycans.