Stereoselective biotransformation and risk assessment of chiral 6PPDQ in the aquatic ecosystems with...

The prevalent emerging contaminant 6PPD-quinone (6PPDQ) in aquatic environments has attracted great attention due to its acute mortality in coho salmon when exposed to urban runoff. It cannot be ignored that 6PPDQ exists as a pair of enantiomers (R-6PPDQ and S-6PPDQ), which may undergo enantioselective transformation in aquatic organisms and cause more complex ecological risks. However, a clear chiral molecular mechanism is lacking for the 6PPDQ, hindering the accurate assessment of the risks of 6PPDQ to aquatic organisms. Herein, this study investigated the precision stereoselective transformation of rac-/R-/S-6PPDQ through experiments and computations. Mass spectrometry results identified different types of hydroxylation metabolites, where significant non-racemization (EF = 0.6) enantiomer-selective transformation was observed. Subsequently, density functional theory (DFT) calculations were performed to explore the potential biotransformation pathways of phenyl oxidation (e.g., ortho- and para- positions) and alkyl oxidation (e.g., C₃₀ and C₄₀ positions). Importantly, there was a nearly 7 kcal/mol relative barrier difference between the rate-determining steps of alkyl-C40 hydroxyl R-6PPDQ and S-6PPDQ, calculated by the accurate quantum mechanical/molecular mechanical (QM/MM) method to consider the active center model and the enzyme environment simultaneously. The overall metabolism presented an interesting conclusion that R-6PPDQ was more easily catalyzed and metabolized than S-6PPDQ. These findings first elucidate the stereoselective bio-metabolism of chiral 6PPDQ by using a synergy strategy, which is helpful for us to develop metabolites as biomarkers for tracking, monitoring, predicting and effectively controlling contaminants in aquatic ecosystems, laying the foundation for accurately assessing the ecological risks of emerging contaminants.