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

Abstract

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.

Oxidative Damage of S‐Containing Amino Acids by the Environmental Radical NO3.: A Kinetic, Product and Computational Study

“…[35] On the other hand, the reaction of NO 3 * with the sulfone Ac-MetO 2 -OMe has a three orders of magnitude lower rate coefficient of 1.3 × 10 6 M À 1 s À 1 (entry 6), which is in the same range as for acyclic aliphatic amino acids and which proceeds through HAT or ET at the secondary amide nitrogen from the backbone. [26] The UV/vis absorption spectrum did not show formation of a reaction transient (Figure S5).…”

“…All kinetic experiments were performed without deoxygenating the samples. [26] It should be noted that slow bleaching of the yellow CAN solution was observed upon mixing with the Met solutions. This finding is in line with the reported oxidation of Met to MetO by Ce(iv), which occurs with a rate coefficient of about 2.5 M À 1 s À 1 (in aqueous media).…”

“…Thus, HAT from the CÀ H bonds adjacent to S or SO, respectively, as well as from the α-CÀ H group in the backbone cannot compete with the barrierless and highly exothermic O-transfer in the respective CT-complexes. On the other hand, in the case of methionine sulfone (Ac-MetO 2 -OMe), where further oxidation of the sulfur atom is not possible, the reaction with NO 3 * occurs at the backbone through a much slower HAT from the α-carbon and ET at the amide, [26] which is predicted to be more favourable than from the CÀ H groups next to the SO 2 moiety (Figure S16).…”

“…[20c,e,25] In reactions of NO 3 * with aliphatic acyclic amino acid residues, ET at the secondary amide moiety was identified as an important reaction pathway, in addition to HAT from activated CÀ H bonds from the side chains or from the α-carbon atom. [26] Reaction of proline residues with NO 3 * occurs predominantly through rapid ET at the tertiary amide motif. [27] The high susceptibility of Met and Cys to oxidation should make these amino acid residues important targets for oxidative attack by environmental NO 3 * .…”

“…Acetonitrile has proven to be a highly suitable solvent for such purposes due to its low reactivity with both NO 3 * and radical intermediates formed in these reactions. [25][26][27] We found previously that having first a detailed understanding of the fundamental reactivity under model conditions is essential to minimise misinterpretation of findings from experiments performed subsequently in more complex environments, particularly in aqueous systems. [20b] This work reveals that reaction of NO 3 * with S-containing amino acid residues is not only extremely fast but also suggests that an irreversible, environmentally driven pathway to methionine sulfone formation and cystine oxidation could exist, which might be relevant for highly polluted environments where the antioxidant defence shield in the ASF could be overwhelmed.…”