Liquid chromatography electrospray ionization Fourier transform ion cyclotron resonance mass spectrometric characterization of N-linked glycans and glycopeptides.

We combine liquid chromatography, electrospray ionization, and Fourier transform ion cyclotron resonance mass spectrometry (LC ESI FT-ICR MS) to determine the sugar composition, linkage pattern, and attachment sites of N-linked glycans. N-linked glycans were enzymatically released from glycoproteins with peptide N-glycosidase F, followed by purification with graphitized carbon cartridge solid-phase extraction and separation over a TSK-Gel Amide80 column under hydrophilic interaction chromatography (HILIC) conditions. Unique glycopeptide compositions were determined from experimentally measured masses for different combinations of glycans and glycopeptides. The method was validated by identifying four peptides glycosylated so as to yield 12 glycopeptides unique in glycan composition for the standard glycoprotein, bovine alpha-2-HS-glycoprotein. We then assigned a total of 137 unique glycopeptide compositions from 18 glycoproteins from fetal bovine serum, and the glycan structures for most of the assigned glycopeptides were heterogeneous. Highly accurate FT-ICR mass measurement is essential for reliable identification.