An electron spin resonance study of electron reactions with peptides: Competitive mechanisms: Deamination vs protonation

Abstract

The reactions of electrons with a number of peptides containing aliphatic and aromatic side groups were investigated in aqueous glasses at low temperatures by electron spin resonance spectroscopy. For the tripeptides glycylalanylalanine, and glycylglycylalanine in neutral glasses (12M LiCl), we find only N-terminal (primary) deamination, at 77 K. The deamination reaction is followed by abstraction from the parent compound. However, abstraction from the N-terminal residue is not observed. In basic glasses evidence for secondary deamination is found. The results found from the study of ten di- and tri-peptides containing one histidyl, phenylalanyl or tyrosyl residues show that electron attachment to the aromatic group effectively competes with the peptide bonds. About 30–40% of the radicals observed are due to primary deamination and the remaining fraction is due to protonated anion of the aromatic side group. As the number of residues increase in the peptide a decrease in attack on the aromatic group is found, with a corresponding increase in primary deamination. The results presented in this work are compared to recent spin trapping and pulse radiolysis investigations on similar systems.