Oxidation of Amino Acids, Peptides, and Proteins by Chlorine Dioxide. Implications for Water Treatment

Abstract

Amino acids, proteins, and peptides occur ubiquitously in natural waters. Although these nitrogen compounds are not directly toxic, harmful byproducts can be formed by reactions with chemical oxidants during water treatment processes. Chlorination and chloramination of water that contains amino acids and other naturally occurring organic matter indeed produces toxic substances, often referred to as disinfection byproducts. The main advantage of using ClO2 over other chlorine containing disinfectants is that the formation of harmful trihalomethanes (THMs), such as chloroform, during water treatment processes is minimized. Because ClO2 is an effective and promising alternative to other chlorine containing disinfectants, the chemistry of ClO2 interactions between amino acids, proteins, and peptides should be thoroughly understood, to ensure the safety of potable water supplies. This chapter presents an overview of the aqueous chemistry of ClO2 and its reactivity with amino acids, peptides, and proteins. The kinetics and products of the reactions are reviewed. Only a few amino acids have been reported to be reactive with ClO2, and have been found to obey first-order kinetics with respect to the concentration of each reactant. The second-order rate constants vary from 10−2 to 107 M−1 s−1 with an order of reactivity: cysteine > tyrosine > tryptophan > histidine > proline. Cysteine sulfonic acid and cystine are products of the oxidation of cysteine by ClO2 in acidic and basic media, respectively. In reactions of histidine, tryptophan, and tyrosine with ClO2, different products are obtained depending on the molar ratios of ClO2 and the individual amino acid. Products also vary depending on whether oxygen is present in the reaction mixture. With excess molar amounts of ClO2 relative to amino acids, low molecular weight compounds are obtained. The oxidation of bovine serum albumin and glucose-6-phosphate dehydrogenase (G6PD) by ClO2 suggests the denaturing of proteins through attack on tryptophan and tyrosine residues. This information is especially important with respect to an understanding of the inactivation of microbes by ClO2.