Are the radical centers in peptide radical cations mobile?: The generation, tautomerism, and dissociation of isomeric α-carbon-centered triglycine radical cations in the gas phase

The mobility of the radical center in three isomeric triglycine radical cations-[G(center dot)GG](+), [GG(center dot)G](+), and [GGG(center dot)](+)-has been investigated theoretically via density functional theory (DFT) and experimentally via tandem mass spectrometry. These radical cations were generated by collision-induced dissociations (CIDs) of Cu(II)-containing ternary complexes that contain the tripeptides YGG, GYG, and GGY, respectively (G and Y are the glycine and tyrosine residues, respectively). Dissociative electron transfer within the complexes led to observation of [Y(center dot)GG](+), [GY(center dot)G](+), and [GGY(center dot)](+); CID resulted in cleavage of the tyrosine side chain as p-quinomethide, yielding [G(center dot)GG](+), [GG(center dot)G](+), and [GGG(center dot)](+), respectively. Interconversions between these isomeric triglycine radical cations have relatively high barriers (>= 44.7 kcal/mol), in support of the thesis that isomerically pure [G(center dot)GG](+), [GG(center dot)G](+), and [GGG(center dot)](+) can be experimentally produced. This is to be contrasted with barriers < 17 kcal/mol that were encountered in the tautomerism of protonated triglycine [Rodriquez C. F. et al. J. Am. Chem. Soc. 2001, 123, 3006-3012]. The CID-spectra of [G(center dot)GG](+), [GG(center dot)G](+), and [GGG(center dot)](+) were substantially different, providing experimental proof that initially these ions have distinct structures. DFT calculations showed that direct dissociations are competitive with interconversions followed by dissociation.