Formation, isomerization, and dissociation of a-carbon-centered and π-centered glycylglycyltryptophan radical cations

Gas phase fragmentations of two isomeric radical cationic tripeptides of glycylglycyltryptophan-[G^•GW]⁺ and [GGW] ^•+-with well-defined initial radical sites at the a-carbon atom and the 3-methylindóle ring, respectively, have been studied using collision-induced dissociation (CID), density functional theory (DFT), and Rice-Ramsperger-Kassel-Marcus (RRKM) theory. Substantially different low-energy CID spectra were obtained for these two isomeric GGW structures, suggesting that they did not interconvert on the time scale of these experiments. DFT and RRKM calculations were used to investigate the influence of the kinetics, stabilities, and locations of the radicals on the competition between the isomerization and dissociation channels. The calculated isomerization barrier between the GGW radical cations (> 35.4 kcal/mol) was slightly higher than the barrier for competitive dissociation of these species (