Reaction of methionine with chlorine: kinetics, product formation, and potential use as a scavenger in chlorine dioxide-based systems

Abstract

The present study investigates the reaction of methionine with free available chlorine (FAC) and estimates the usability of methionine as a selective scavenger for intrinsically formed FAC in chlorine dioxide (ClO₂)-based reactions. It has been reported that ClO₂ forms chlorite and FAC in the reaction with phenolic compounds. However, for some reactive moieties such as reduced sulfur-containing compounds, no quantification of FAC could be carried out yet, due to the limitation of the current methods which arise from the high reactivity of these compounds with both ClO₂ and FAC. Methionine reacts fast with FAC (k_app = 6.8 × 10⁸ M⁻¹ s⁻¹ at pH 7), and very slow with ClO₂ (k_app = 10⁻² M⁻¹ s⁻¹ at pH 7). Methionine sulfoxide and chloride are formed in equal parts in the reaction of methionine with FAC, as stable products. Hence the yield of methionine sulfoxide and chloride can principally be used to quantify intrinsically formed FAC. While the results for phenol were in accordance with the literature (50% chlorite and 50% FAC), hydroquinone and 1,4-dimethylpiperazine also resulted in methionine sulfoxide yields, even though no FAC formation has been reported for these compounds. It is possible that reactive organic intermediates are formed, which additionally causes methionine sulfoxide formation (e.g., semiquinone or phenoxy radicals). Based on this study, the FAC formation in the reaction of glutathione with ClO₂ can be confirmed and is expected to be 50%. However, since methionine sulfoxide can be formed by reactive organic intermediate species, quantitative statements should be handled with care.