Inverse hydrogen migration in arginine-containing peptide ions upon electron transfer

Collisional electron transfer from gaseous Cs atoms was studied for singly and doubly protonated peptides Gly-Arg (GR) and Ala-Arg (AR) at 50- and 100-keV kinetic energies. Singly protonated GR and AR were discharged to radicals that in part rearranged by migration of a C_α hydrogen atom onto the guanidine group. The C_α-radical isomers formed were detected as stable anions following transfer of a second electron. In addition to the stabilizing rearrangements, the radicals underwent side-chain and backbone dissociations. The latter formed z fragments that were detected as the corresponding anions. Analysis of the (GR+H)^· radical potential energy surface using electronic structure theory in combination with Rice-Ramsperger-Kassel-Marcus calculations of rate constants indicated that the arginine C_α hydrogen atom was likely to be transferred to the arginine side-chain on the experimental timescale of ≤200 ns. Transfer of the Gly C_α-H was calculated to have a higher transition-state energy and was not kinetically competitive. Collisional electron transfer to doubly protonated GR and AR resulted in complete dissociation of (GR+2H)^+· and (AR+2H)^+· ions by loss of H, ammonia, and N-C_α bond cleavage. Electronic structure theory analysis of (GR+2H)^+· indicated the presence of multiple conformers and electronic states that differed in reactivity and steered the dissociations to distinct channels. Electron attachment to (GR+2H)²⁺ resulted in the formation of closely spaced electronic states of (GR+2H)^+· in which the electron density was delocalized over the guanidinium, ammonium, amide, and carboxyl groups. The different behavior of (GR+H)^· and (GR+2H)^+· is explained by the different timescales for dissociation and different internal energies acquired upon electron transfer.