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Lasso peptides constitute a class of bioactive peptides sharing a knottedstructure where the C-terminal tail of the peptide is threaded through andtrapped within an N-terminalmacrolactamring. The structural characterization oflasso structures and differentiation from their unthreaded topoisomers is nottrivial and generally requires the use of complementary biochemical andspectroscopic methods. Here we investigated two antimicrobial peptidesbelonging to the class II lasso peptide family and their correspondingunthreaded topoisomers: microcin J25 (MccJ25), which is known to yieldtwo-peptide product ions specific of the lasso structure under collisioninduceddissociation (CID), and capistruin, for which CID does not permit tounambiguously assign the lasso structure. The two pairs of topoisomers wereanalyzed by electrospray ionization Fourier transform ion cyclotron resonancemass spectrometry (ESI-FTICR MS) upon CID, infrared multiple photondissociation (IRMPD), and electron capture dissociation (ECD). CID andECDspectra clearly permitted to differentiate MccJ25 from its non-lassotopoisomer MccJ25-Icm, while for capistruin, only ECD was informative andshowed different extent of hydrogen migration (formation of c\bullet/z fromc/z\bullet) for the threaded and unthreaded topoisomers. The ECD spectra of thetriply-charged MccJ25 and MccJ25-lcm showed a series of radical b-type productions {\eth}b0In{\TH}. We proposed that these ions are specific ofcyclic-branched peptides and result from a dual c/z\bullet and y/bdissociation, in the ring and in the tail, respectively. This work shows thepotentiality of ECD for structural characterization of peptide topoisomers, aswell as the effect of conformation on hydrogen migration subsequent to electroncapture.

Topoisomer Differentiation of Molecular Knots by FTICR MS: Lessons from Class II Lasso Peptides