Multistep collisionally activated decomposition in an ion trap for the determination of the amino-acid sequence and gas-phase ion chemistry of lithium-coordinated valinomycin

Evidence from collisionally activated decomposition (CAD) of electrospray-p roduced ions in an ion trap shows that lithium ion binds to the backbone es ter oxygen atoms of valinomycin to open the cyclodepsipeptide ring at the D -alpha-hydroxyvaleric acid and L-lactic acid residues. The two resulting ri ng-opened, linear acylium ions are sequenced by multiple stages (up to MS10 ) of CAD. Amino-acid residues are sequentially cleaved from the acylium ter minus of the peptides, one amino-acid residue at each stage of the CAD expe riment, until each acylium ion is converted to a tripeptide species at MS10 . This method builds upon a previously published strategy for determining t he amino-acid sequences of cyclic peptides and is used here for the valinom ycin-based class of ionophore antibiotics. This entirely instrumental appro ach overcomes ambiguities encountered in assigning amino-acid sequences of cyclic peptides by other tandem mass spectrometric methods. These ambiguiti es arise from indiscriminate and multiple ring-opening reactions that occur during collisional activation of cyclic peptides, resulting in tandem mass spectra that are superpositions of random fragment ions. Multiple stages o f CAD in an ion trap also facilitate more accurate interpretation of the ta ndem mass spectra of valinomycin [M + Li](+) by unambiguously revealing the genealogies of the fragment ions. Furthermore, it reveals new gas-phase al dehyde elimination, proton transfer, and intramolecular ion rearrangement r eactions that occur upon collisional activation of valinomycin [M + Li](+). Cyclic peptides produce b(n) ions upon ring opening and fragmentation. The refore, they may serve as models for understanding the mechanisms of linear peptide fragmentation. (Int J Mass Spectrom 194 (2000) 247-259) (C) 2000 E lsevier Science B.V.