Extended Data Figure 2: Structure elucidation by multistage tandem and electrospray ionization high-resolution mass spectrometry.

a, A single-stage high-resolution MS/MS experiment revealed a superposition of fragment ions typically found for cyclic peptides. Quasi molecular ion selected for fragmentation is marked with a blue rhombus (m/z = 783.45). For sequence annotation, fragment ions were searched for b-ions of high intensity. One fragmentation route is annotated exemplarily by highlighting respective a-, b- and c-ion series signals. Thia, thiazoldine. b, Data generated by multistage tandem mass spectrometry show that lugdunin mainly fragmented along four routes (blue digits; (I), (II), (III), (IV)). Initial protonation occurs at the secondary amine of thiazolidine (blue H atom). Proton transfer to nearby peptide bonds initiated ring cleavage with subsequent fragmentation. Initial loss of ammonia for fragmentation route (I) can be explained by a precedent six-membered transition state. Intensities are in arbitrary units. Blue rhombuses label the position of initial ring cleavage. Fragmentation route molecules are shown in linearized form.