Sequencing of O-Glycopeptides Derived from an S-Layer Glycoprotein of Geobacillus stearothermophilus NRS 2004/3a Containing up to 51 Monosaccharide Residues at a Single Glycosylation Site by Fourier Transform Ion Cyclotron Resonance Infrared Multiphoton Dissociation Mass Spectrometry

The microheterogeneity of large sugar chains in glycopeptides from S-layer glycoproteins containing up to 51 monosaccharide residues at a single O-attachment site on a 12 amino acid peptide backbone was investigated by Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS). Structural elucidation of glycopeptides with the same amino acid sequence and different glycoforms, having such a high saccharide-to-peptide ratio, was achieved by applying infrared multiphoton dissociation (IRMPD) MS/MS for the first time. A 100% sequence coverage of the glycan chain and a 50% coverage of the peptide backbone fragmentation were obtained. The microheterogeneity of carbohydrate chains at the same glycosylation site, containing largely rhamnose, could have been reliably assessed.