Figure 3.

LC–MS/MS data for xanthoxycyclin A. A, extracted ion chromatogram at m/z 649.371. Inset left: mass spectrum at the chromatogram peak. Inset right, cyclic peptide sequence with MS/MS ring cleavage point shown by an arrow, expected m/z (m/z_exp) and observed m/z (m/z_obs). B, MS/MS spectrum of protonated molecule at m/z 649.371. A strong sequence of b ions is derived from initial ring opening between Leu and Phe residues. Immonium ions are labeled with the corresponding amino acid residue.