Cascade dissociations of peptide cation-radicals. Part 2. Infrared multiphoton dissociation and mechanistic studies of z-ions from pentapeptides.

ABSTRACT: Dissociations of z(4) ions from pentapeptides AAXAR where X=H, Y, F, W, and V produce dominant z(2) ions that account for >50 % of the fragment ion intensity. The dissociation has been studied in detail by experiment and theory and found to involve several isomerization and bond-breaking steps. Isomerizations in z(4) ions proceed by amide trans?cis rotations followed by radical-induced transfer of a ?-hydrogen atom from the side chain, forming stable C(?) radical intermediates. These undergo rate-determining cleavage of the C(?)-CO bond at the X residue followed by loss of the neutral AX fragment, forming x(2) intermediates. The latter were detected by energy-resolved resonant excitation collision-induced dissociation (CID) and infrared multiphoton dissociation (IRMPD) experiments. The x(2) intermediates undergo facile loss of HNCO to form z(2) fragment ions, as also confirmed by energy-resolved CID and IRMPD MS(4) experiments. The loss of HNCO from the x(2) ion from AAHWR is kinetically hampered by the Trp residue that traps the OCNH radical group in a cyclic intermediate.

SUBMITTER: Ledvina AR 

PROVIDER: S-EPMC3619414 | biostudies-literature | 2012 Aug

REPOSITORIES: biostudies-literature