Methylamidation for isomeric profiling of sialylated glycans by nanoLC-MS

The analysis of isomeric glycans is a challenging task. In this work, a new strategy was developed for isomer-specific glycan profiling using nanoLC-MS with PGC as the stationary phase. Native glycans were derivatized in the presence of methylamine and trispyrrolidinophosphonium hexafluorophosphate and reduced by the ammonia-borane complex. Methylamidation stabilized the retention time and peak width and improved the detection sensitivity of sialylated glycans to 2-80-fold in comparison to previous ESI-MS methods using the positive-ion mode. Up to 19 tetrasialylated glycan species were identified in the derivatized human serum sample, which were difficult to detect in the sample without derivatization. Furthermore, due to high detection sensitivity and chromatographic resolution, more isomeric glycans could be identified from the model glycoprotein Fetuin and the human serum sample. As a result, up to seven isomers were observed for the disialylated biantennary glycan released from Fetuin, and three of them were identified for the first time in this study. Using the developed analytical strategy, a total of 293 glycan species were obtained from the human serum sample, representing an increase of over 100 peaks in comparison to the underivatized sample. The strategy greatly facilitates the profiling of isomeric glycans and the analysis of trace-level samples.