RADICAL CATIONS OF AMINO ACIDS AND PEPTIDES: STRUCTURES AND STABILITIES

Amino acid and peptide radical cations, M center dot+, are formed by oxidative dissociations of [Cu(auxiliary ligand)(M)](center dot 2+) and [Metal(III)(salen)(M)](+) complexes. The most easily formed radicals contain either air aromatic or basic amino acid residue. Aromatic amino acids have ion; ionization energies, are easily oxidized and delocalize the charge and spin over the ring systems; basic amino acids facilitate formation of alpha-radicals that have captodative structures in which the charge and spin are formally separated, although feeding back some of the charge onto the amide ate carboxyl group adjacent to the radical center through hydrogen bonding enriches tire electron-withdrawing properties and is highly stabilizing. DFT calculations located five isomers of His(center dot+) with an alpha-radical with a captodative structure at the global minimum in a deep potential well. An IRMPD spectrum confirmed that this isomer is the experimentally observed long-lived isomer When both charge and spin are oil the peptide backbone, as in [GGG](center dot+), captodative structures have the lowest energies; the barriers to interconversion between the three isomeric alpha-radicals of [GGG](center dot+) are high as the charge impedes migration of a hydrogen atom. Dissociation of [GGG](center dot+) is charge-driven. h? peptide radical cations containing a basic atnino acid residue the charge is sequestered oil the side chain and the radical center, either on the backbone or oil another side chain, initiates the fragmentation. (C) 2009 Wiley Periodicals, Inc., Mass Spec Rev 28:655-671, 2009