On-line selective enrichment and ion-pair reaction for structural determination of sulfated glycopeptides by capillary electrophoresis–mass spectrometry

We describe a new capillary electrophoresis–mass spectrometry (CE–MS)-based technique for analyzing sulfated glycopeptides. The proposed method performs selective enrichment of sulfated glycopeptides from a complex mixture of peptides based on field-enhanced sample injection and ion-pair reaction with a basic ion-pair reagent (Lys-Lys-Lys; KKK) at the exit end of a capillary in a single analysis, which permits successful fragmentation of sulfated glycopeptides in positive-ion mode at the MS/MS stage for comprehensive structural analysis. In this study, the method was verified using a model sulfated monosaccharide, N-acetyl-d-galactosamine 4-sulfate (GalNAc 4S). As an example of an application of this method, sulfated glycopeptides were selectively enriched from the enzymatic digest of thyroid stimulating hormone, affording ∼500-fold sensitivity enhancement, and structural information was successfully obtained via on-line ion-pair complexation reaction.