| 研究実績の概要 |
We've successfully observed collision-induced isomerization in anabaenopeptins, a group of macrocyclic peptides from Anabaena sp. cyanobacteria. Generation of fragment ions from macrocyclic structures inevitably require bond cleavages at two positions in the cyclic system. In the collision-induced dissociation process of macrocyclic compounds, precursor ions may be isomerized to intermediate ions in which the first position has been cleaved but the second position is intact. Anabaenopeptins share a common macrocyclic structure consist of five amino acid residues but each analogue has a different amino acid connected to an exocyclic position through ureido linkage. The difference between anabaenopeptin A (AP-A) and B (AP-B) is the exocyclic amino acid residue; the former has Tyr whereas the latter has Arg. They showed striking differences in the spectral patterns; AP-A showed many fragment ions attributable to the cleavages of macrocyclic structure whereas AP-B showed fragment ions attributable to the cleavages of exocyclic part only. Interestingly, the ion mobility (IM) spectra of AP-A revealed the presence of isomeric [M+H]+ ion structures prior to fragmentation. Energy-resolved IM/MS/MS experiments showed that relative abundance of the isomeric ion structures depends on the degree of collisional excitation prior to the IM separation; the isomeric ions structures were generated by collision-induced isomerization. No isomeric [M+H]+ was observed for AP-B, which does not show cleavages in the macrocyclic structure.
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