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

Abstract

Evidence from collisionally activated decomposition (CAD) of electrospray-produced ions in an ion trap shows that lithium ion binds to the backbone ester oxygen atoms of valinomycin to open the cyclodepsipeptide ring at the D-α-hydroxyvaleric acid and L-lactic acid residues. The two resulting ring-opened, linear acylium ions are sequenced by multiple stages (up to MS 10) of CAD. Amino-acid residues are sequentially cleaved from the acylium terminus of the peptides, one amino-acid residue at each stage of the CAD experiment, until each acylium ion is converted to a tripeptide species at MS 10. This method builds upon a previously published strategy for determining the amino-acid sequences of cyclic peptides and is used here for the valinomycin-based class of ionophore antibiotics. This entirely instrumental approach overcomes ambiguities encountered in assigning amino-acid sequences of cyclic peptides by other tandem mass spectrometric methods. These ambiguities 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 of CAD in an ion trap also facilitate more accurate interpretation of the tandem mass spectra of valinomycin [M + Li] + by unambiguously revealing the genealogies of the fragment ions. Furthermore, it reveals new gas-phase aldehyde elimination, proton transfer, and intramolecular ion rearrangement reactions that occur upon collisional activation of valinomycin [M + Li] +. Cyclic peptides produce b n ions upon ring opening and fragmentation. Therefore, they may serve as models for understanding the mechanisms of linear peptide fragmentation.