Radicals containing the histidine residue have been generated in the gas phase by femtosecond electron transfer to protonated histidine-N-methylamide (1H(+)), N-alpha-acetylhistidine-N-methylamide (2H(+)), N-alpha-glycylhistidine (3H(+)), and N-alpha-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 N-C-alpha 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 N-C-alpha 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 Moller-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 N-alpha-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.