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 β-hydrogen atom from the side chain, forming stable C(β) radical intermediates. These undergo rate-determining cleavage of the C(α)-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.
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Aaron R. Ledvina, Thomas W. Chung, Renjie Hui, Joshua J. Coon, Frantisek Turecek. Cascade dissociations of peptide cation-radicals. Part 2. Infrared multiphoton dissociation and mechanistic studies of z-ions from pentapeptides.. Journal of The American Society for Mass Spectrometry, Springer Verlag (Germany), 2012, 23 (8), pp.1351-1363. ⟨10.1007/s13361-012-0409-8⟩. ⟨hal-00880184⟩

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