Studying the photochemical transformation mechanism of dissolved free amino acids (DFAA) in chromophoric dissolved organic matter (CDOM) solution facilitates the understanding of DFAA's environmental fate and bioavailability change upon solar irradiation in natural surface waters. Tyrosine oxidation product (Tyr-OH) was synthesized to quantify the primary transformation product (tyrosine peroxide, Tyr-OOH) in CDOM solution. Both reactions between superoxide radical anion (O2-) and tyrosine radical (Tyr) and between singlet oxygen (1O2) and tyrosine (Tyr) yield Tyr-OOH, which is subsequently transformed into Tyr-OH. The reaction between O2- and Tyr not only generated Tyr-OOH but also caused the regeneration of Tyr. O2- and 1O2 contributed 30-44% to Tyr's transformation in CDOM solutions at pH 8.0, in which 1O2 oxidation accounted for 6-11%. The contribution of O2- to Tyr's phototransformation process was the difference between the total contribution of O2- and 1O2 and the individual contribution of 1O2. Compared with the fast assimilation of Tyr, Tyr-OH was stable in natural water under dark incubation, indicating that phototransformation decreased the bioavailability of Tyr.
Keywords: Amino acids; Dissolved organic matter; Peroxide; Superoxide radical anion; Tyrosine.
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