Fig. 5.

Evidence for glycan O-linkage at serine residues. The ETD spectrum of the m/z 877.52+ precursor confirms the identity of the PilA peptide ¹⁰¹VSSGAITW¹⁰⁸ and identifies serine 103 as the site of pentasaccharide occupancy. Abundance (in arbitrary units) is shown on the y axis. Its location to this residue is derived by detection of fragment ions c3+ (VSS) to c7+ (VSSGAIT) at m/z 1,224.4 to 1,566.5, respectively, representing five c-ions of the c-ion series c6+ to c11+ with a mass increase of 934.2 Da, which corresponds to the complete pentasaccharide glycan residue. Furthermore, the c2+ (VS) fragment ion at m/z 204.0 was detected only in its glycan-free form.