Density Functional Theory Study of Semiquinone Radical Anions of Polychlorinated Biphenyls in the Syn- and Anti-like Conformation

Abstract

Polychlorinated biphenyls can be metabolized to reactive metabolites, such as PCB semiquinone radical anions (SQ•−), whose structure and role in PCB-induced toxicity are difficult to investigate due to their relative instability. The unrestricted UB3LYP/6-311G** method was used to investigate several molecular descriptors of the syn- and anti-like conformation of SQs•−. The bond lengths and angles of the quinone moiety of the SQs•− were in between the values reported for PCB quinones and hydroquinones, which is consistent with the distribution of the α-HOMO. The dihedral angles between the two ring systems increased in the presence of ortho chlorine substituents and were smaller compared to the corresponding PCB quinones. The ground state energies indicate that the anti-like conformation of the SQs•− is more favorable than the syn-like conformation. Molecular descriptor used for modeling of quantitative structure-activity relationships displayed some dependence on the conformation. These findings suggest that SQs•− in both the syn- and anti-like conformation may interact differently with target molecules, which may have implications for the toxicity of PCBs.