A model assessment of the role played by the carbonate (CO3·-) and dibromide (Br2·-) radicals in the photodegradation of glutathione in sunlit fresh- and salt-waters

Glutathione (GLU) is a peptidic thiol that plays important anti-oxidant roles in organisms and that occurs in both freshwater and seawater, where it can undergo both bio- and photodegradation. Recent results have elucidated the role played by ·OH, 1O2, H2O2 and other yet unidentified transients in GLU photochemistry, but very little is known of the role of CO3·-. This is an important gap because CO3·- is usually very reactive towards electron-rich compounds including thiols and mercaptans. Very little is also known on the environmental importance of the reaction between GLU and Br2·-, which could account for the literature finding that GLU phototransformation is enhanced in simulated seawater compared to freshwater. By means of a photochemical model approach based on the APEX software (Aqueous Photochemistry of Environmentally-occurring Xenobiotics), here we provide an assessment of the role that several photoreactants, including most notably CO3·- and Br2·-, have in the photodegradation of GLU (both the whole substance and the separate neutral and mono-anionic species) under representative fresh- and saltwater conditions. Our model suggests that CO3·- would dominate the photodegradation of GLU in low-DOC and high-pH freshwater. They are the only freshwater conditions that really ensure GLU photodegradation to be competitive with biotransformation. In surface seawater and in brackish water, GLU phototransformation would be dominated by the Br2·- reaction.