Radicals Formed in N-Acetylproline by Electron Attachment: Electron Spin Resonance Spectroscopy and Computational Studies

Abstract

In this study, the reactions of electrons with N-acetylproline are investigated by electron spin resonance (ESR) spectroscopy and density functional theory. Electrons are produced by γ irradiation or by photoionization of K(4)Fe(CN)(6) in neutral 7.5 M LiCl-D(2)O aqueous glasses at low temperatures with identical results. Electrons are found to add to both the peptide bond and the carboxyl group of the acetyl-proline moiety at 77 K. On annealing, both the electron adducts undergo fragmentation of the peptide bond between the nitrogen and the α carbon of the peptide structure. However, the peptide bond electron adduct radical reacts much more rapidly than the carboxyl group electron adduct radical. The DFT calculations predict that the carboxyl adduct is substantially more stable than the peptide bond adduct, with the activation barrier to N-Cα cleavage 3.7 kcal/mol for the amide electron adducts and 23 kcal/mol for the carboxyl electron adducts in inagreement with the relative reactivity found by experiment.