An Explanation for the 32 and 54 Da Losses Seen in N-Linked Glycan Permethylation Reactions

Permethylation is a widely used derivatization technique for mass spectrometry-based glycan analysis, applicable to both research and drug product characterization. It stabilizes sialic acids, improves glycan ionization, and is commonly used to analyze PNGase F-released N-linked glycans for distribution and composition profiling, yielding semiquantitative or quantitative data. We occasionally observed peaks 32 and 54 Da below expected ion masses, anomalies which are unreported in the literature. Investigation revealed these losses stem from slower-than-expected spontaneous exchange of C-1 amines for water in nascent glycosylamines following PNGase F release. Here, we report the origin of these mass losses, propose a mechanistic explanation, and suggest mitigation strategies.