title: Probing peptide cation-radicals by near-UV photodissociation in the gas phase. Structure elucidation of histidine radical chromophores formed by electron transfer reduction.
abstract: Electron transfer reduction of gas-phase ions generated from histidine-containing peptides forms stable cation-radicals that absorb light at 355 nm, as studied for AAHAR, AAHAK, DSHAK, FHEK, HHGYK, and HHSHR. Laser photodissociation of mass-selected cation-radicals chiefly resulted in loss of H atoms, contrasting dissociations induced by slow collisional heating. The 355 nm absorption was due to new chromophores created by electron transfer and radical rearrangements in the cation-radicals. The chromophores were identified by time-dependent density functional theory calculations as 2H,3H-imidazoline and 2H-dihydrophenol radicals, formed by hydrogen atom transfer to the histidine and tyrosine side chain groups, respectively. These radicals undergo facile C-H bond dissociations upon photon absorption. In contrast, dissociations of histidine peptide cation-radicals containing the 1H,3H-imidazoline ring prefer loss of 4-methylimidazole via a multistep reaction pathway. The isomeric cation-radicals can be distinguished by a combination of collision-induced dissociation and near-UV photodissociation. The TD-DFT excitation energies in model imidazoline radicals were benchmarked on EOM-CCSD energies, and a satisfactory agreement was found for the M06-2X and ωB97XD functionals. The combination of electron transfer, photodissociation, collisional activation, and theory is presented as a powerful tool for studying structures and electronic properties of peptide cation-radicals in the gas phase.
authors: Department of Chemistry, University of Washington , Bagley Hall, Box 351700, Seattle, Washington 98195-1700, United States.
authors: Huong T H
authors: HT
authors: Nguyen
authors: Christopher J
authors: CJ
authors: Shaffer
authors: František
authors: F
authors: Tureček
mesh_terms: D000595:Amino Acid Sequence; D002412:Cations / Q000737:chemistry; D004579:Electron Transport; D005609:Free Radicals / Q000737:chemistry*; D005740:Gases / Q000737:chemistry*; D006639:Histidine / Q000737:chemistry; D007093:Imidazoles / Q000737:chemistry; D010455:Peptides / Q000737:chemistry / Q000378:metabolism*; D010782:Photolysis / Q000528:radiation effects; D013816:Thermodynamics; D014466:Ultraviolet Rays
chemical_list: D002412:Cations; D005609:Free Radicals; D005740:Gases; D007093:Imidazoles; D010455:Peptides; D006639:Histidine; C023439:4-methylimidazole