Chlorination increases the persistence of semiquinone free radicals derived from polychlorinated biphenyl hydroquinones and quinones.
Polychlorinated biphenyls (PCBs) comprise a group of persistent organic pollutants that differ significantly in their physicochemical properties, their persistence, and their biological activities. They can be metabolized via hydroxylated and dihydroxylated metabolites to PCB quinone intermediates. We have recently demonstrated that both dihydroxy PCBs and PCB quinones can form semiquinone radicals (SQ(*-)) in vitro. These semiquinone radicals are reactive intermediates that have been implicated in the toxicity of lower chlorinated PCB congeners. Here we describe the synthesis of selected PCB metabolites with differing degrees of chlorination on the oxygenated phenyl ring, e.g., 4,4'-dichloro-biphenyl-2,5-diol, 3,6,4'-trichloro-biphenyl-2,5-diol, 3,4,6,-trichloro-biphenyl-2,5-diol, and their corresponding quinones. In addition, two chlorinated o-hydroquinones were prepared, 6-chloro-biphenyl-3,4-diol and 6,4'-dichloro-biphenyl-3,4-diol. These PCB (hydro-)quinones readily react with oxygen or via comproportionation to yield the corresponding semiquinone free radicals, as detected by electron paramagnetic resonance spectroscopy (EPR alias ESR). The greater the number of chlorines on the (hydro-)quinone (oxygenated) ring, the higher the steady-state level of the resulting semiquinone radical at near neutral pH.
Song
Yang
Y
Department of Occupational and Environmental Health, The University of Iowa, 100 Oakdale Campus, 124 IREH, Iowa City, Iowa 52242-5000, USA.
Buettner
Garry R
GR
Parkin
Sean
S
Wagner
Brett A
BA
Robertson
Larry W
LW
Lehmler
Hans-Joachim
HJ
D016227:Benzoquinones / Q000138:chemical synthesis*; D004578:Electron Spin Resonance Spectroscopy; D004785:Environmental Pollutants / Q000378:metabolism; D054879:Halogenation; D006873:Hydroquinones / Q000737:chemistry*; D010100:Oxygen / Q000737:chemistry; D011078:Polychlorinated Biphenyls / Q000737:chemistry* / Q000378:metabolism; D011809:Quinones / Q000737:chemistry*
D016227:Benzoquinones; D004785:Environmental Pollutants; D006873:Hydroquinones; D011809:Quinones; C025232:semiquinone radicals; D011078:Polychlorinated Biphenyls; D010100:Oxygen