Enantioseparation and identification for the rationalization of the environmental impact of 4 polychlorinated biphenyls

Differences in Enantioselective Hydroxylation of 2,2',3,6-Tetrachlorobiphenyl (CB45) and 2,2',3,4',6-Pentachlorobiphenyl (CB91) by Human and Rat CYP2B Subfamilies

Although polychlorinated biphenyls (PCBs) were commercially banned half a century ago, contamination of the environment and organisms by PCBs is still observed. PCBs show high persistence and bioaccumulation, resulting in toxicity. Among PCBs, chiral PCBs with more than three chlorine atoms at the ortho-position exhibit developmental and neurodevelopmental toxicity. Because toxicity is dependent on the atropisomer, atropisomer-specific metabolism is vital in determining toxicity. However, structural information on enantioselective metabolism remains elusive. Cytochrome P450 (CYP, P450) monooxygenases, particularly human CYP2B6 and rat CYP2B1, metabolize separated atropisomers of 2,2',3,6-tetrachlorobiphenyl (CB45) and 2,2',3,4',6-pentachlorobiphenyl (CB91) to dechlorinated and hydroxylated metabolites. Docking studies using human CYP2B6 predict 4'-hydroxy (OH)-CB45 from (aR)-CB45 as a major metabolite of CB45. Di-OH- and dechlorinated OH-metabolites from human CYP2B6 and rat CYP2B1 are also detected. Several hydroxylated metabolites are derived from CB91 by both P450s; 5-OH-CB91 is predicted as a major metabolite. CB91 dechlorination is also detected by identifying 3-OH-CB51. A stable conformation of PCBs in the substrate-binding cavity and close distance to P450 heme are responsible for high metabolizing activities. As hydroxylation and dechlorination change PCB toxicity, this approach helps understand the possible toxicity of chiral PCBs in mammals.

A proposed method of enantioselectivity analysis for residual chiral PCBs in gas chromatography

Polychlorinated biphenyls (PCBs) are harmful and persistent organic pollutants. The influence of chiral PCBs acts mainly on the different enantiomer fraction, bioaccumulation and even degradation in environmental media. Solvents and temperatures existed almost everywhere during the analysis of extraction, purification, concentration and detection, which was often underestimated in previous studies. In our study, the configuration stability of the chiral PCBs was examined from solvent and temperature aspects. Transformation phenomena for the analytic stereoisomer monomers of PCB45, PCB95, and PCB149 affected by temperature were observed. We demonstrated that higher inlet temperatures could increase the sensibility for the low-concentration environmental samples, resulting in isomerization of chiral PCBs. Real rice samples were used to verify our analysis method. Combined with density functional theory, we verified the mechanism of isomer conversion with various numbers and sites of the -Cl substituent. PCBs with tetra-ortho substituents (2, 2', 6, 6') were relatively stable and showed the highest rotational barriers (E_a) at approximate 240 kJ mol^-1. Others with trio-ortho substituents (2, 2', 6/6') showed E_a from 170 to 190 kJ mol^-1, whose enantiomeric fractions would be affected by temperature during the analysis process for environmental detection. The method we developed was a promising means to understand the mechanism of isomerization and to predict stabilities of chiral PCBs.