Oxidative Damage in Aliphatic Amino Acids and Di- and Tripeptides by the Environmental Free Radical Oxidant NO₃^•: The Role of the Amide Bond Revealed by Kinetic and Computational Studies

Kinetic and computational data reveal a complex behavior of the important environmental free radical oxidant NO₃^• in its reactions with aliphatic amino acids and di- and tripeptides, suggesting that attack at the amide N–H bond in the peptide backbone is a highly viable pathway, which proceeds through a proton-coupled electron transfer (PCET) mechanism with a rate coefficient of about 1 × 10⁶ M^–1 s^–1 in acetonitrile. Similar rate coefficients were determined for hydrogen abstraction from the α-carbon and from tertiary C–H bonds in the side chain. The obtained rate coefficients for the reaction of NO₃^• with aliphatic di- and tripeptides suggest that attack occurs at all of these sites in each individual amino acid residue, which makes aliphatic peptide sequences highly vulnerable to NO₃^•-induced oxidative damage. No evidence for amide neighboring group effects, which have previously been found to facilitate radical-induced side-chain damage in phenylalanine, was found for the reaction of NO₃^• with side chains in aliphatic peptides.