Anaerobic Microbial Transformation of Halogenated Aromatics and Fate Prediction Using Electron Density Modeling

Halogenated homo- and heterocyclic aromatics including disinfectants, pesticides and pharmaceuticals raise concern as persistent and toxic contaminants with often unknown fate. Remediation strategies and natural attenuation in anaerobic environments often build on microbial reductive dehalogenation. Here we describe the transformation of halogenated anilines, benzonitriles, phenols, methoxylated, or hydroxylated benzoic acids, pyridines, thiophenes, furoic acids, and benzenes by Dehalococcoides mccartyi strain CBDB1 and environmental fate modeling of the dehalogenation pathways. The compounds were chosen based on structural considerations to investigate the influence of functional groups present in a multitude of commercially used halogenated aromatics. Experimentally obtained growth yields were 0.1 to 5 × 10¹⁴ cells mol^–1 of halogen released (corresponding to 0.3–15.3 g protein mol^–1 halogen), and specific enzyme activities ranged from 4.5 to 87.4 nkat mg^–1 protein. Chlorinated electron-poor pyridines were not dechlorinated in contrast to electron-rich thiophenes. Three different partial charge models demonstrated that the regioselective removal of halogens is governed by the least negative partial charge of the halogen. Microbial reaction pathways combined with computational chemistry and pertinent literature findings on Co^I chemistry suggest that halide expulsion during reductive dehalogenation is initiated through single electron transfer from B₁₂Co^I to the apical halogen site.