Fig. 2

O-GlcNAc peptide ionization is suppressed in presence of unmodified peptides. A GlcNAcylated peptide YSPTgSPSK ([M+GlcNAc+H]⁺ 1069.5) and its unmodified form ([M+H]⁺ 866.4) are mixed at indicated molar ratios and analyzed by MALDI-TOF using α-cyano-4-hydroxycinnamic acid as the matrix. Arrows point to peaks of m/z 1069.5. B GlcNAcylated peptides coelute with their unmodified forms on reverse-phase HPLC. RF column is packed with a 10-cm-length of C18 beads. Gradient is 5–40% solvent B (solvent A: 0.1% formic acid; solvent B: 90% acetonitrile, 0.1% formic acid) in 60 min at a flow rate of 300 nl/min. Extracted ion chromatograms are shown. Time of elution peaks is as indicated. C GlcNAcylated peptide YSPTgSPSK ([M+GlcNAc+H]⁺ 1069.5) is mixed with its unmodified form ([M+H]⁺ 866.4) at indicated molar ratios and sprayed into LCQ mass spectrometer via electrospray ionization. Averaged full scan spectra are shown. Arrows point to peaks of m/z 1069.5. Peaks of m/z 904.5, 918.3, and 942.5 are due to peptide impurities