The arginine anomaly: Arginine radicals are poor hydrogen atom donors in electron transfer induced dissociations

Arginine amide radicals are generated by femtosecond electron transfer to protonated arginine amide cations in the gas phase. A fraction of the arginine radicals formed (2-amino-5-dihydroguanid-1'-yl-pentanamide, 1H) is stable on the 6.7 mu s time scale and is detected after collisional reionization. The main dissociation of 1H is loss of a guanidine molecule from the side chain followed by consecutive dissociations of the 2-aminopentanamid-5-yl radical intermediate. Intramolecular hydrogen atom transfer from the guanidinium group onto the amide group is not observed. These results are explained by ab initio and density functional theory calculations of dissociation and transition state energies. Loss of guanidine from 1H is calculated to require a transition state energy of 68 kJ mol(-1), which is substantially lower than that for hydrogen atom migration from the guanidine group. The loss of guanidine competes with the reverse migration of the arginine alpha-hydrogen atom onto the guanidyl radical. RRKM calculations of dissociation kinetics predict the loss of guanidine to account for > 95% of 1H dissociations. The anomalous behavior of protonated arginine amide upon electron transfer provides an insight into electron capture and transfer dissociations of peptide cations containing arginine residues as charge carriers. The absence of efficient hydrogen atom transfer from charge-reduced arginine onto sterically proximate amide group blocks one of the current mechanisms for electron capture dissociation. Conversely, charge-reduced guanidine groups in arginine residues may function as radical traps and induce side-chain dissociations. In light of the current findings, backbone dissociations in arginine-containing peptides are predicted to involve excited electronic states and proceed by the amide superbase mechanism that involves electron capture in an amide pi* orbital, which is stabilized by through-space coulomb interaction with the remote charge carriers.