Cascade dissociations of peptide cation-radicals. Part 2. Infrared multiphoton dissociation and mechanistic studies of z-ions from pentapeptides.
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.
Department of Chemistry, University of Wisconsin, Madison, WI 53706, USA.
Aaron R
AR
Ledvina
Thomas W
TW
Chung
Renjie
R
Hui
Joshua J
JJ
Coon
Frantisek
F
Tureček
D002412:Cations / Q000737:chemistry; D007536:Isomerism; D007700:Kinetics; D008958:Models, Molecular; D009842:Oligopeptides / Q000737:chemistry*; D053719:Tandem Mass Spectrometry; D013816:Thermodynamics; D014364:Tryptophan / Q000737:chemistry
D002412:Cations; D009842:Oligopeptides; D014364:Tryptophan