Formation mechanisms of chlorinated disinfection byproducts chlorinated benzoquinones from free aromatic amino acids: In vitro and in silico study

Chlorinated benzoquinones (CBQs) are a class of disinfection byproducts (DBPs) that are widely present in aquatic environments. Due to their strong cytotoxicity and genotoxicity to aquatic organisms and humans, the identification of novel CBQs and the exploration of their formation mechanisms became the focus of recent research. In this study, a new group of chlorinated DBPs formed during chlorination of two aromatic amino acids, tyrosine (Tyr) and tryptophan (Trp), were identified for the first time. Four predominant formation pathways of the main chlorinated product from Tyr and Trp, dichloro-benzoquinone (DCBQ), were deduced based on the MASS spectrum data, which consist of reactions including electrophilic addition, chlorination, hydrolysis, decarboxylation, oxidation, dehydrochlorination, and cleavage of the pyrrole ring. The potential chlorination substituted sites of Tyr to form 2,6-DCBQ were further explored by quantum chemical calculations. Chlorination tended to occur at the ortho C-position of the –OH (hydroxyl) group in Tyr. The C1 site of Tyr was most prone to undergo monochlorination, and dichlorination was more likely to take place at C5 site of the monochlorinated product. In addition, the nitrogen atom (N18) of Tyr could also undergo chlorine substitution, but it was difficult to happen when the C1 and C5 sites were already substituted. By combining experimental and theoretical results, this study summarized the predominant pathway for the formation of 2,6-DCBQ during the chlorination process of amino acids. The outcome contributed to a deeper understanding of the implications for water safety and the potential health risks associated with chlorination byproducts.