Transformation of amino acid tyrosine in chromophoric organic matter solutions: Generation of peroxide and change of bioavailability.

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 (O) and tyrosine radical (Tyr) and between singlet oxygen (O) and tyrosine (Tyr) yield Tyr-OOH, which is subsequently transformed into Tyr-OH. The reaction between O and Tyr not only generated Tyr-OOH but also caused the regeneration of Tyr. O and O contributed 30-44% to Tyr's transformation in CDOM solutions at pH 8.0, in which O oxidation accounted for 6-11%. The contribution of O to Tyr's phototransformation process was the difference between the total contribution of O and O and the individual contribution of O. 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.