Computational mapping of CYP450 metabolism for Benzotriazole UV absorbers: Biotransformation pathways, mechanisms and environmental implications demonstrated with UV-P

Benzotriazole UV stabilizers (BT-UVs) are an emerging class of organic pollutants notoriously marked for the environmental ubiquity, bioaccumulativity, and toxicological impacts on human health. Metabolic biotransformation via cytochrome P450 represents a key detoxification route, yet the underlying biotransformation mechanisms of BT-UVs remain a critical knowledge gap. Under the catalysis of enzymatic core structure (Cpd I) in P450, the biotransformation of the environmentally-prevalent congener, i.e. 2-(2'-hydroxy-5'-methylphenyl)-benzotriazole (UV-P) was theoretically mimicked using density functional theory (DFT). Except the regioselectivity and two-state reactivity of Cpd I, a wide range of biotransformation pathways and the underlying chemical mechanisms were unraveled in a systematic way. Overall, the higher reactivity of Cpd I in doublet was consistently shown to be higher than that in quartet. Hydrogen abstraction from 2-hydroxyl was prioritized with the remarkably lower activation energies (0.93-1.13 kcal·mol^-1) and higher reaction rates (10⁻¹⁹ cm³·molec^-1·s^-1), while the subsequent hydroxylation via nearly-barrierless rebound of hydroxyl would be rapidly initiated at the neighboring aromatic carbon. The aromatic electrophilic additions regioselectively oriented at para- and methyl/hydroxyl-substituted sites of phenyl ring were inferior to hydrogen abstraction. Compared with the secondary epoxidation and NIH, the proton shuttle and rebound steps occurring after electrophilic addition were more privileged for hydroxylation. Despite tardiness in kinetics, the relatively favorable oxidation at the pyridinic N-site was shown as a rationale for the possible occurrence of nitrated products in organisms. Although the less acute or chronic toxicities were roughly predicted for variety of products, the multi-level toxicological investigation or long-term surveillance in vivo was suggested in the future. Unequivocally, the theoretical insights into metabolic biotransformation would establish a mechanistic framework for well understanding biotransformation fate and toxicological complexity of BT-UVs.