Generation, Characterization, and Dissociation of Radical Cations Derived from Prolyl-glycyl-glycine

Radical cations of an aliphatic tripeptide prolyl-glycyl-glycine (PGG^•+) and its sequence ions [a₃ + H]^•+ and [b₂ - H]^•+ have been generated by collision-induced dissociation of the [Cu(Phen)(PGG)]^•2+ complex, where Phen = 1,10-phenanthroline. Infrared multiple photon dissociation spectroscopy, ion-molecule reaction experiments, and theoretical calculations have been used to investigate the structures of these ions. The unpaired electron in these three radical cations is located at different α-carbons. The PGG^•+ radical cation has a captodative structure with the radical at the α-carbon of the proline residue and the proton on the oxygen of the first amide group. This structure is at the global minimum on the potential energy surface (PES). By contrast, the [a₃ + H]^•+ and [b₂ - H]^•+ ions are not the lowest-energy structures on their respective PESs, and their radicals are formally located at the C-terminal and second α-carbons, respectively. Density functional theory calculations on the structures of the ternary copper(II) complex ion suggest that the charge-solvated isomer of the metal complex is the precursor ion that dissociates to give the PGG^•+ radical cation. The isomer of the complex in which PGG is bound as a zwitterion dissociates to give the [a₃ + H]^•+ and [b₂ - H]^•+ ions.