title: Histidine-containing radicals in the gas phase.
abstract: Radicals containing the histidine residue have been generated in the gas phase by femtosecond electron transfer to protonated histidine-N-methylamide (1H+), Nalpha-acetylhistidine-N-methylamide (2H+), Nalpha-glycylhistidine (3H+), and Nalpha-histidylglycine (4H+). Radicals generated by collisional electron transfer from dimethyldisulfide to ions 1H+ and 2H+ at 7 keV collision energies were found to dissociate completely on the microsecond time scale, as probed by reionization to cations. The main dissociations produced fragments from the imidazole side chain and the cleavage of the C(alpha)CO bond, whereas products of NCalpha bond cleavage were not observed. Electron transfer from gaseous potassium atoms to ions 3H+ and 4H+ at 2.97 keV collision energies not only caused backbone NCalpha bond dissociations but also furnished fractions of stable radicals that were detected after conversion to anions. Ion structures, ion-electron recombination energies, radical structures, electron affinities, and dissociation and transition-state energies were obtained by combined density functional theory and Møller-Plesset perturbational calculations (B3-PMP2) and basis sets ranging from 6-311+G(2d,p) to aug-cc-pVTZ. The Rice-Ramsperger-Kassel-Marcus theory was used to calculate rate constants on the B3-PMP2 potential energy surfaces to aid interpretation of the mass spectrometric data. The stability of Nalpha-histidylglycine-derived radicals is attributed to an exothermic isomerization in the imidazole ring, which is internally catalyzed by reversible proton transfer from the carboxyl group. The isomerization depends on the steric accessibility of the histidine side chain and the carboxyl group and involves a novel cation radical-COO salt-bridge intermediate.
authors.affiliation: Department of Chemistry, University of Washington, Bagley Hall, Box 351700, Seattle, Washington 98195, USA. turecek@chem.washington.edu
authors.forename: Frantisek
authors.initials: F
authors.lastname: Turecek
authors.forename: Chunxiang
authors.initials: C
authors.lastname: Yao
authors.forename: Y M Eva
authors.initials: YM
authors.lastname: Fung
authors.forename: Shigeo
authors.initials: S
authors.lastname: Hayakawa
authors.forename: Mami
authors.initials: M
authors.lastname: Hashimoto
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Matsubara
mesh_terms: D000577:Amides / Q000737:chemistry; D005609:Free Radicals / Q000737:chemistry; D005740:Gases / Q000737:chemistry*; D006639:Histidine / Q000737:chemistry*; D007536:Isomerism; D007700:Kinetics; D008958:Models, Molecular; D008968:Molecular Conformation; D011789:Quantum Theory; D013816:Thermodynamics
chemical_list: D000577:Amides; D005609:Free Radicals; D005740:Gases; D006639:Histidine