Phenols are ubiquitous environmental pollutants, whose biotransformation involving phenol coupling catalyzed by cytochromes P450, may produce more lipophilic and toxic metabolites. DFT computations were performed to explore the debated phenol coupling mechanisms, taking triclosan as a model substrate. We find that a diradical pathway facilitated by Compound I and protonated Compound II of P450 is favored vs. alternative radical-addition or electron-transfer mechanisms. The identified diradical coupling resembles a "two-state reactivity" from Compound I characterized by significantly high rebound barriers of the phenoxy radicals, which can be formulated into three equations for calculating the ratio [coupling]/[hydroxylation]. A higher rebound barrier than H-abstraction for triclosan in the high-spin state can facilitate the phenoxy radical dissociation and thus to enable phenol coupling, while H-abstraction/radical-rebound causing phenol hydroxylation via minor rebound barriers mostly occur in the low-spin state. Therefore, oxidation of triclosan by P450 fits the first equation with a ratio [coupling]/[hydroxylation] of 1:4, consistent with experimental data indicating different extents of triclosan coupling (6-40%). The high rebound barrier of phenoxy radicals, as a key for the mechanistic identification of phenol coupling vs. hydroxylation, originates from their weak electron donor ability due to spin aromatic delocalization. We envision that the revealed mechanism can be extended to the cross-coupling reactions between different phenolic pollutants, and the coupling reactions of several other aromatic pollutants, to infer unknown metabolites.
Guo, F., Chai, L., Zhang, S., Yu, H., Liu, W., Kepp, K. P., & Ji, L. (2020). Computational Biotransformation Profile of Emerging Phenolic Pollutants by Cytochromes P450: Phenol Coupling Mechanism. Environmental Science and Technology, 54(5), 2902-2912. https://doi.org/10.1021/acs.est.9b06897