Chirality can be exploited to gain insight into enantioselective fate processes that may otherwise remain undetected because only biological, but not physical and chemical transport and transformation processes in an achiral environment will change enantiomer compositions. This review provides an in-depth overview of the application of chirality to the study of chiral polychlorinated biphenyls (PCBs), an important group of legacy pollutants. Like other chiral compounds, individual PCB enantiomers may interact enantioselectively (or enantiospecifically) with chiral macromolecules, such as cytochrome P-450 enzymes or ryanodine receptors, leading to differences in their toxicological effects and the enantioselective formation of chiral biotransformation products. Species and congener-specific enantiomer enrichment has been demonstrated in environmental compartments, wildlife and mammals, including humans, typically due to a complex combination of biotransformation processes and uptake via the diet by passive diffusion. Changes in the enantiomer composition of chiral PCBs in the environment have been used to understand complex aerobic and anaerobic microbial transformation pathways, to delineate and quantify PCB sources and transport in the environment, to gain insight into the biotransformation of PCBs in aquatic food webs, and to investigate the enantioselective disposition of PCBs and their methylsulfonyl PCBs metabolites in rodents. Overall, changes in chiral signatures are powerful, but currently underutilized tools for studies of environmental and biological processes of PCBs.

Changes in atropisomer composition of chiral polychlorinated biphenyls (PCBs) and their mono- and di- hydroxylated metabolites (OH- and diOH-PCBs) via rat cytochrome P450 2B1 (CYP2B1) mediated biotransformation were investigated in vitro. Rat CYP2B1 could stereoselectively biotransform chiral PCBs to generate meta-OH-PCBs as the major metabolites after 60 min incubations. Non-racemic enantiomer fractions (EFs: concentration ratios of the (+)-atropisomer or the first-eluting atropisomer over the total concentrations of two atropisomers) of 5-OH-PCBs, were 0.17, 0.20, 0.85, 0.77 and 0.41 for incubations with PCBs 91, 95, 132, 136 and 149, respectively. CYP-mediated stereoselective formation of diOH-PCBs from OH-PCBs was observed for the first time. After 60 min stereoselective biotransformation, the EFs of both 4-OH-PCB 95 and 5-OH-PCB 95 changed from racemic (i.e., 0.50) to 0.62 and 0.46, respectively. These transformations generated statistically non-racemic 4,5-diOH-PCB 95, with EFs of 0.53 and 0.58 for 4-OH-PCB 95 and 5-OH-PCB 95 incubations, respectively. Biotransformation of PCBs 91 and 136 also generated 4,5-diOH-PCB 91 and 4,5-diOH-PCB 136, respectively. These in vitro results were consistent with that observed for stereoselective PCB biotransformation by rat liver microsomes and in vivo. Biotransformation interference between two atropisomers of PCB 136 was investigated for the first time in this study. The biotransformation process of (−)-PCB 136 was significantly disrupted by the presence of (+)-PCB 136, but not the other way around. Thus, stereoselective metabolism of chiral PCBs and OH-PCBs by CYPs is a major mechanism for atropisomer composition change of PCBs and their metabolites in the environment, with the degree of composition change dependent, at least in part, on stereoselective interference of atropisomers with each other at the enzyme level.

Substituted diphenylamine antioxidants (SDPAs) and benzotriazole UV stabilizers (BZT-UVs), previously under reported classes of organic contaminants, were determined in sediment, water, and freshwater biota in an urban creek in Canada. SDPAs and BZT-UVs were frequently detected in all matrices including upstream of the urban area in a rural agricultural/woodlot region, suggesting a ubiquitous presence and bioaccumulation of these emerging contaminants. Spatial comparisons were characterized by higher levels of SDPAs downstream compared with the upstream, implying a possible influence of the urban activities on the antioxidant contamination in the sampling area. In sediment, 4,4'-bis(α,α-dimethylbenzyl)-diphenylamine (diAMS), dioctyl-diphenylamine (C8C8), and dinonyl-diphenylamine (C9C9) were the most dominant congeners of SDPAs, with concentrations up to 191 ng/g (dry weight, d.w.). Benthic invertebrates Crayfish (Orcoescties spp.) had larger body burdens of SDPAs and BZT-UVs compared to pelagic fish (hornyhead chub (Nocomis biguttatus) and common shiner (Luxilus cornutus)) in the creek and partitioning coefficients demonstrated that sediment was the major reservoir of these contaminants. This is the first report of bioaccumulation and partitioning behaviors of SDPAs and BZT-UVs in freshwater environments.