Production Of OH-PBDEs From Laccase-Catalyzed Oxidation Of 2,4-DBP And 2,4,6-TBP

Polybrominated diphenyl ethers (PBDEs) are a group of brominated flame retardants that have emerged as persistent organic contaminants of particular concern because of their ubiquitous distribution, persistence, toxicity and bioaccumulation potential. Hydroxylated polybrominated diphenyl ethers (HO-PBDEs) have been frequently found in the marine biosphere as emerging organic contaminants. Studies to date have suggested that HO-PBDEs in marine biota are natural products. However, the mechanisms leading to the biogenesis of HO-PBDEs are still far from clear. In this study, using a laccase isolated from Trametes versicolor as the model enzyme, we explored the formation of HO-PBDEs from the laccase-catalyzed oxidation of simple bromophenols (e.g.,2,4-DBP and 2,4,6-TBP).The main research results:(1) We studied the reaction kinetics of the laccase-catalyzed conversion of 2-BP and 2,6-DBP. The results showed that the laccase-catalyzed conversion of the two bromophenols followed a first-order kinetics model. A higher the concentration of laccase enhanced bromophenols conversion rate. The reaction was pH-dependant and a faster conversion rate was obtained around pH 5.0. A higher conversion of bromophenols could be achieved at a higher temperature within 5-45℃.(2) We Studied the reaction pathway and mechanism of laccase-catalyzed conversion of 2,4-DBP and 2,4,6-TBP. Experiments under ambient conditions clearly showed that HO-PBDEs were readily produced from 2,4-DBP and 2,4,6-TBP in presence of laccase. Polybrominated compounds 2’-OH-BDE68, 2,2’-diOH-BB80, and 1,3,8-TrBDD were identified as the products from 2,4-DBP, and 2’-HO-BDE121 and 4’-HO-BDE121 from,4,6-TBP. The production of HO-PBDEs was likely a result of the coupling of bromophenoxy radicals, generated from the laccase-catalyzed oxidation of 2,4-DBP or 2,4,6-TBP. The transformation of bromophenols by laccase was pH-dependant, and was also influenced by enzymatic activity. All the identified products increased initially and the decreased gradually, suggesting the further oxidation of these compounds by laccase. In view of the abundance of 2,4-DBP and 2,4,6-TBP and the phylogenetic distribution of laccases in the environment, laccase-catalyzed conversion of bromophenols may be potentially an important route for the natural biosynthesis of HO-PBDEs.