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

Abstract

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.

Role of Direct and Sensitized Photolysis in the Photomineralization of Dissolved Organic Matter and Model Chromophores to Carbon Dioxide

“…Figure C suggests that the photomineralization of umbelliferone is oxygen-dependent, while flavone is oxygen independent. Umbelliferone is likely undergoing direct photolysis mediated by oxygen as an electron acceptor, as suggested by other studies, or perhaps is undergoing sensitized reactions with 1 O 2 or another ROS (e.g., O 2 •– ). Conversely, flavone is being degraded through an oxygen independent pathway such as the formation and reaction with a triplet.…”

Development of an Elementary Reaction-Based Kinetic Model to Predict the Aqueous-Phase Fate of Organic Compounds Induced by Reactive Free Radicals

“…The simulated time-dependent concentration profiles of each species were compared with the experimental results. Here, our previous studies demonstrated two examples of elementary reaction-based kinetic models, as follows: (1) acetone induced by HO • in a UV/H 2 O 2 AOP and (2) methionine induced by PPRIs in the presence of two surrogate CDOMs (i.e., 1,4-naphthoquinone and 2-napthaldehyde) in sunlit fresh waters …”

“… MohrhardtB.BarriosB.KiblerR.KingW.DoskeyP. V.MinakataD. Mohrhardt, B. Barrios, B. Kibler, R. King, W. Doskey, P. V. Minakata, D. 10.1021/acs.est.3c041762023571436314372 An elementary reaction-based kinetic model based on experimental measurements and quantum chemical computations of the photochemical fate of methionine in the presence of surrogate dissolved organic matter was developed and validated. …”

“…Overall reaction pathways and yields (%) of transformation products in the presence of 1,4-naphthoquinone (red) and 2-naphthaldehyde (blue) (reproduced from ref ; copyright 2023 American Chemical Society).…”