Elucidation of the Photochemical Fate of Methionine in the Presence of Surrogate and Standard Dissolved Organic Matter under Sunlight Irradiation.

The abiotic fate of dissolved free amino acids considerably contributes to the cycling of dissolved sulfur and nitrogen in natural aquatic environments. However, the roles of the functional groups of chromophoric dissolved organic matter (CDOM) and the fate of free amino acids under sunlight irradiation in fresh waters are not fully understood. This study aims to elucidate the fate of photolabile methionine in the presence of three CDOM surrogate compounds, i.e., 1,4-naphthoquinone, 2-naphthaldehyde, and umbelliferone, and two standard CDOM by coupling experimental measurement, quantum chemical computations, and kinetic modeling. Results indicate that excited triplet-state CDOM and hydroxyl radicals are able to cleave the C-S bond in methionine, resulting in the formation of smaller amino acids and volatile sulfur-containing compounds. Singlet oxygen forms methionine sulfoxide and methionine sulfone. The distribution of phototransformation products offers an improved understanding of the fate of nitrogen- and sulfur-containing compounds and their uptake by microorganisms in natural aquatic environments.