Laccase-Catalyzed Transformation Of Bromophenols To Polybrominated Diphenyl Ethers (OH-PBDEs)

Hydroxylated polybrominated diphenyl ethers (OH-PBDEs), a new class of emerging endocrine disrupting chemicals, have become an environmental concern because of their higher toxicity than PBDEs and the possibility of transformation to brominated dioxins under natural conditions. Many studies have revealed that OH-PBDEs can be the metabolites of PBDEs or naturally synthetic products in the living organisms. However, natural formation of these compounds is poorly understood. In the study, we explored laccase-catalyzed transformation of simple bromophenols (BPs, e.g.,4-BP,2,4-DBP, and2,4,6-TBP) to OH-PBDEs. The main results include:(1) We explored the reaction kinetics of the transformation of4-BP,2,4-DBP, and2,4,6-TBP catalyzed by laccase. The results showed that the transformation of BPs followed pseudo first-order kinetics. There appeared a linear correlation between the pseudo first-order kinetic constants and the concentration within12h, when the initial concentrations of bromophenols were fixed at5.00umol/L. When the initial concentrations of the laccase were kept at12.80U/mL, a linear correlation between the pseudo first-order kinetic constants and the concentration of laccase can be found within12h. When the concentration changed with the laccase/bromophenol ratio constant, the conversion rate increased with the increase of concentrations of both chemicals.(2) The effects of the environmental conditions (e.g., pH and temperature) on the transformation reactions of BPs were studied. Results showed that the transformation of bromophenols was faster at lower pH in the range of5to8. The transformation rates of the bromophenols at each temperature between15℃and45℃followed the order2,4-DBP>2,4,6-TBP>4-BP at pH7.0. The activation energies of laccase-mediated transformation of these compounds were calculated to be37.07kJ/mol for4-BP,19.18kJ/mol for2,4-DBP, and24.00kJ/mol for2,4,6-DBP.(3) The formation products were identified and the possible pathways were proposed. Three OH-PBDEs,6-OH-BDE-13,2’-OH-BDE-68and2’-OH-BDE-121, were produced from the laccase-catalyzed transformation of4-BP,2,4-DBP and2,4,6-DBP separately. In addition,2,4-DBP and2,4,6-TBP could be oxidized to form2-bromohydroquinone and2,6-dibrmohydroquinone. Bromohydroquinone was not found for4-BP. The formation of OH-PBDEs may result from the oxidative coupling reactions between bromophenoxy radicals generated from lose of electrons to the laccase. Given the widespread BPs and laccases, oxidation of BPs by laccases may be a potential pathway for the production of OH-PBDEs in the natural environment.