Institute of Groundwater Ecology , Helmholtz Zentrum München , Ingolstädter Landstr. 1 , 85764 Neuherberg , Germany.
Chair of Analytical Chemistry and Water Chemistry , Technical University of Munich , Marchioninistrasse 17 , D-81377 Munich , Germany.
Environ Sci Technol. 2019 Mar 5;53(5):2332-2343. doi: 10.1021/acs.est.8b03717. Epub 2019 Feb 20.
Trichloromethane (TCM) is a frequently detected and persistent groundwater contaminant. Recent studies have reported that two closely related Dehalobacter strains (UNSWDHB and CF) transform TCM to dichloromethane, with inconsistent carbon isotope effects (εC = -4.3 ± 0.45‰; εC = -27.5 ± 0.9‰). This study uses dual element compound specific isotope analysis (C; Cl) to explore the underlying differences. TCM transformation experiments using strain CF revealed pronounced normal carbon and chlorine isotope effects (εC = -27.9 ± 1.7‰; εCl = -4.2 ± 0.2‰). In contrast, small carbon and unprecedented inverse chlorine isotope effects were observed for strain UNSWDHB (εC = -3.1 ± 0.5‰; εCl = 2.5 ± 0.3‰) leading to opposing dual element isotope slopes (λ = 6.64 ± 0.14 vs λ = -1.20 ± 0.18). Isotope effects of strain CF were identical to experiments with TCM and Vitamin B (εC = -26.0 ± 0.9‰, εCl = -4.0 ± 0.2‰, λ = 6.46 ± 0.20). Comparison to previously reported isotope effects suggests outer-sphere-single-electron transfer or S2 as possible underlying mechanisms. Cell suspension and cell free extract experiments with strain UNSWDHB were both unable to unmask the intrinsic KIE of the reductive dehalogenase (TmrA) suggesting that enzyme binding and/or mass-transfer into the periplasm were rate-limiting. Nondirected intermolecular interactions of TCM with cellular material were ruled out as reason for the inverse isotope effect by gas/water and gas/hexadecane partitioning experiments indicating specific, yet uncharacterized interactions must be operating prior to catalysis.
三氯甲烷（TCM）是一种经常检测到的持久性地下水污染物。最近的研究报告称，两种密切相关的脱硫弧菌（UNSWDHB 和 CF）将 TCM 转化为二氯甲烷，碳同位素效应不一致（εC = -4.3 ± 0.45‰；εC = -27.5 ± 0.9‰）。本研究使用双元素化合物特定同位素分析（C；Cl）来探索潜在的差异。使用 CF 菌株进行 TCM 转化实验表明，存在明显的正常碳和氯同位素效应（εC = -27.9 ± 1.7‰；εCl = -4.2 ± 0.2‰）。相比之下，UNSWDHB 菌株观察到较小的碳和前所未有的反氯同位素效应（εC = -3.1 ± 0.5‰；εCl = 2.5 ± 0.3‰），导致相反的双元素同位素斜率（λ = 6.64 ± 0.14 vs λ = -1.20 ± 0.18）。CF 菌株的同位素效应与 TCM 和维生素 B 的实验相同（εC = -26.0 ± 0.9‰，εCl = -4.0 ± 0.2‰，λ = 6.46 ± 0.20）。与先前报道的同位素效应相比，表明可能的机制是外球单电子转移或 S2。用 UNSWDHB 菌株进行细胞悬液和无细胞提取物实验均未能揭示还原脱卤酶（TmrA）的固有 KIE，表明酶结合和/或质量转移到周质是限速步骤。通过气体/水和气体/十六烷分配实验排除了 TCM 与细胞物质之间非定向分子间相互作用是反同位素效应的原因，这表明在催化之前必须存在特定但尚未表征的相互作用。
