Oxidation of Amino Acids, Peptides and Proteins by Ozone: A Review

The kinetics and products of the reaction of ozone with specific amino acids, peptides, and proteins are reviewed based on studies reported in the literature. Ozone reacts mainly with the unprotonated amino group of the acids and the second-order ozone rate constants for these reactions, except for cysteine, methionine, and tryptophan, vary by about two-orders from 2.6 × 104 to 4.4 × 106 M−1s−1. The site of attack on cysteine and methionine by O3 is at the sulfhydryl rather than the amino group to give sequential O-atom addition products. The order of reactivity for the oxidation of amino acids by O3 at pH 8 is cysteine > tryptophan ≈ methionine > phenylalanine ≈ histidine > others, with half-lives mostly in the range of milliseconds to tens of seconds (1 mg L-1 O3 dose). Reactions of O3 with aliphatic amino acids form nitrate, ammonia, and one or two carbon atom-containing carbonyl and carboxylic byproducts. In the ozonolysis of peptides and proteins, oxidation by O3 occurs at the tyrosine, tryptophan, histidine, cysteine, and methionine residues. Oxidation of proteins results in changes in their folding ability and tertiary structures.