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 Cα hydrogen atom onto the guanidine group. The Cα-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 Cα hydrogen atom was likely to be transferred to the arginine side-chain on the experimental timescale of ≤200 ns. Transfer of the Gly CαH 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 NCα 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.

Graphical AbstractElectron transfer in arginine-containing peptides induces inverse H-atom migrations from the Cα position to the guanidine group.Figure optionsDownload full-size imageDownload high-quality image (89 K)Download as PowerPoint slide