FIGURE 11.

LC/MS analyses performed on β-propiolactone-treated peptide 7. LC/MS chromatograms demonstrated two reaction products (I and II) with a single and double β-propiolactone attachment. Ia, the MS² spectrum obtained from peptide 7 with a single attachment of β-propiolactone provided only a fragment ion (406.8) with a neutral loss of 120 Da. Ib, multistage activation performed on the fragment ion (406.8) revealed the peptide sequence with a truncated methionine residue (−48 Da). The second product (II) with two attachments of β-propiolactone resulted in a comparable (Ia) MS² spectrum and (Ib) multistage fragmentation spectrum.