Figure 4.
Low energy ESI-FTICR-MS/MS spectra of cysteine containing peptides (CHWKQNDEQM) labeled with 3-maleimidopropionic acid. The sulfur in the side-chain thio-ether residue is converted to sulfoxide by oxidation. The ions at m/z 760.2837 correspond to [M+2H]2+ ions of oxidized modified peptides and were isolated in the quadrupole and subjected to different collisional trap voltage settings. Mass spectra shown were acquired with −3V, −9V, −12V, and −15V (A, B, C, D). *denotes noise peaks.