Absolute configuration of 2,2',3,3',6-pentachlorinatedbiphenyl (PCB 84) atropisomers.

Nineteen polychlorinated biphenyl (PCB) congeners, such as 2,2',3,3',6-pentachlorobiphenyl (PCB 84), display axial chirality because they form stable rotational isomers, or atropisomers, that are non-superimposable mirror images of each other. Although chiral PCBs undergo atropselective biotransformation and atropselectively alter biological processes, the absolute structure of only a few PCB atropisomers has been determined experimentally. To help close this knowledge gap, pure PCB 84 atropisomers were obtained by semi-preparative liquid chromatography with two serially connected Nucleodex β-PM columns. The absolute configuration of both atropisomers was determined by X-ray single-crystal diffraction. The PCB 84 atropisomer eluting first and second on the Nucleodex β-PM column correspond to (aR)-(-)-PCB 84 and (aS)-(+)-PCB 84, respectively. Enantioselective gas chromatographic analysis with the β-cyclodextrin-based CP-Chirasil-Dex CB gas chromatography column showed the same elution order as the Nucleodex β-PM column. Based on earlier reports, the atropisomers eluting first and second on the BGB-172 gas chromatography column are (aR)-(-)-PCB 84 and (aS)-(+)-PCB 84, respectively. An inversion of the elution order is observed on the Cyclosil-B gas chromatography and Cellulose-3 liquid chromatography columns. These results advance the interpretation of environmental and human biomonitoring as well as toxicological studies.