title: The histidine effect. Electron transfer and capture cause different dissociations and rearrangements of histidine peptide cation-radicals.
abstract: Electron-transfer and -capture dissociations of doubly protonated peptides gave dramatically different product ions for a series of histidine-containing pentapeptides of both non-tryptic (AAHAL, AHAAL, AHADL, AHDAL) and tryptic (AAAHK, AAHAK, AHAAK, HAAAK, AAAHR, AAHAR, AHAAR, HAAAR) type. Electron transfer from gaseous Cs atoms and fluoranthene anions triggered backbone dissociations of all four N-C(alpha) bonds in the peptide ions in addition to loss of H and NH(3). Substantial fractions of charge-reduced cation-radicals did not dissociate on an experimental time scale ranging from 10(-6) to 10(-1) s. Multistage tandem mass spectrometric (MS(n)) experiments indicated that the non-dissociating cation-radicals had undergone rearrangements. These were explained as being due to proton migrations from N-terminal ammonium and COOH groups to the C-2' position of the reduced His ring, resulting in substantial radical stabilization. Ab initio calculations revealed that the charge-reduced cation-radicals can exist as low-energy zwitterionic amide pi* states which were local energy minima. These states underwent facile exothermic proton migrations to form aminoketyl radical intermediates, whereas direct N-C(alpha) bond cleavage in zwitterions was disfavored. RRKM analysis indicated that backbone N-C(alpha) bond cleavages did not occur competitively from a single charge-reduced precursor. Rather, these bond cleavages proceeded from distinct intermediates which originated from different electronic states accessed by electron transfer. In stark contrast to electron transfer, capture of a free electron by the peptide ions mainly induced radical dissociations of the charge-carrying side chains and loss of a hydrogen atom followed by standard backbone dissociations of even-electron ions. The differences in dissociation are explained by different electronic states being accessed upon electron transfer and capture.
authors.affiliation: Department of Chemistry, Bagley Hall, Box 351700, University of Washington, Seattle, Washington 98195-1700, USA. turecek@chem.washington.edu
authors.forename: Frantisek
authors.initials: F
authors.lastname: Turecek
authors.forename: Thomas W
authors.initials: TW
authors.lastname: Chung
authors.forename: Christopher L
authors.initials: CL
authors.lastname: Moss
authors.forename: Jean A
authors.initials: JA
authors.lastname: Wyer
authors.forename: Anneli
authors.initials: A
authors.lastname: Ehlerding
authors.forename: Anne I S
authors.initials: AI
authors.lastname: Holm
authors.forename: Henning
authors.initials: H
authors.lastname: Zettergren
authors.forename: Steen Brøndsted
authors.initials: SB
authors.lastname: Nielsen
authors.forename: Preben
authors.initials: P
authors.lastname: Hvelplund
authors.forename: Julia
authors.initials: J
authors.lastname: Chamot-Rooke
authors.forename: Benjamin
authors.initials: B
authors.lastname: Bythell
authors.forename: Béla
authors.initials: B
authors.lastname: Paizs
mesh_terms: D002412:Cations / Q000737:chemistry; D003198:Computer Simulation; D004583:Electrons; D005609:Free Radicals / Q000737:chemistry; D006639:Histidine / Q000737:chemistry*; D010455:Peptides / Q000737:chemistry*; D011487:Protein Conformation
chemical_list: D002412:Cations; D005609:Free Radicals; D010455:Peptides; D006639:Histidine