Mechanistic insight into hydroxylation of 2,2′,4,4′-tetrabromodiphenyl ether during biodegradation by typical aerobic bacteria: Experimental and computational studies

Highlights: • Hydroxyl radical was revealed with great impact on aerobic biodegradation of BDE-47. • Hydroxylations initiated by ∙OH were theoretically studied with DFT computation. • Electrophilic addition at brominated-site may lead preferentially to dihydroxylation. • Hydrogen abstraction was indicated as a favorable path for generation of 5-OH-BDE-47. Polybrominated diphenyl ethers (PBDEs) are a group of persistent pollutants in the environment. Though aerobic biodegradation of PBDEs have been extensively studied, the involved hydroxylation mechanism decisive for whole biotransformation is not clear yet. During the effective biodegradation of 2,2′,4,4′-tetrabromodiphenyl ether (BDE-47) by B. xenovorans LB400, the depletion of endogenous ∙OH by scavenger could bring about the significant decrease of biodegradation efficiency whereas $·O_2^{-}$ was nearly not influential. Given the importance of ∙OH in hydroxylation, the reaction mechanisms along major pathways of electrophilic addition and hydrogen abstraction were theoretically examined by density functional theory (DFT). For the less demand of activation energy, the relative preference of electrophilic addition was shown at aromatic C3-site. When the secondary reaction was considered after addition at C4-site, the barrierless association of ∙OH at C3-site and deprotonation by H₂O was validated as the energetically-favorable pathway that may cause dihydroxylation of BDE-47 into 3,4-dihydroxyl-BDE-17. The electrophilic addition followed by seconary barrierless trans-association of ∙OH and then dehydration seemed favorable for monohydroxylation as regards energetic barrier merely up to 194.01 kJ mol⁻¹, while the hydrogen abstraction by ∙OH from C5-site was more privileged actually. The theoretical insights would help well understand the hydroxylation mechanism of PBDEs by aerobes.