Probing Peptide Cation–Radicals by Near-UV Photodissociation in the Gas Phase. Structure Elucidation of Histidine Radical Chromophores Formed by Electron Transfer Reduction

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.