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)-O-center dot, O-1(2), H2O2 and other yet unidentified transients in GLU photochemistry, but very little is known of the role of CO3 center dot-. This is an important gap because CO3 center dot- 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 Br-2(center dot-), 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 center dot- and Br-2(center dot-), 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 center dot- would dominate the photodegradation of GLU in low-DOC and high-pH freshwater, which are the only freshwater conditions that really ensure GLU photodegradation to be competitive with biotransformation. This result supports the potential key importance of CO3 center dot- in the environmental photochemistry of GLU. In surface seawater and in brackish water, GLU phototransformation might be dominated by the Br-2(center dot-) reaction (the role of additional halogen species such as Cl-2(center dot-) and ClBr(center dot-)is still unknown). (C) 2018 Elsevier Ltd. All rights reserved.