Fig. 4.

Der f 2 tryptic G⁴⁹KPFTLEALFDANQNTK⁶⁵ peptide modified by 0–9 Hex units. A, MS1 averaged over eight full scans showing the nonmodified peptide (m/z 632.33, z = 3) as the most intense precursor ion in the m/z 600–1200 mass range. The sequential mass increments of 54 (z = 3) represent individual Hex units. B, MS1 averaged over nine full scans ∼2.5 min earlier in the LC chromatogram showing the G⁴⁹KPFTLEALFDANQNTK⁶⁵ modified by 5–9 Hex units. The m/z 735–1230 range is amplified ×50 in panel A and B. C, ETD-MS2 spectrum of the precursor ion at m/z 686.35, z = 3 showing the unambiguous identification of Lys⁵⁰ as the modification site. Black circle represents hexose.