Reductive Dehalogenation of Trichloromethane by Two Different Dehalobacter restrictus Strains Reveal Opposing Dual Element Isotope Effects

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 (ε 13 C UNSWDHB = -4.3 ± 0.45‰; ε 13 C CF = -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 (ε 13 C CF = -27.9 ± 1.7‰; ε 37 Cl CF = -4.2 ± 0.2‰). In contrast, small carbon and unprecedented inverse chlorine isotope effects were observed for strain UNSWDHB (ε 13 C UNSWDHB = -3.1 ± 0.5‰; ε 37 Cl UNSWDHB = 2.5 ± 0.3‰) leading to opposing dual element isotope slopes (λ CF = 6.64 ± 0.14 vs λ UNSWDHB = -1.20 ± 0.18). Isotope effects of strain CF were identical to experiments with TCM and Vitamin B 12 (ε 13 C Vitamin B12 = -26.0 ± 0.9‰, ε 37 Cl Vitamin B12 = -4.0 ± 0.2‰, λ Vitamin B12 = 6.46 ± 0.20). Comparison to previously reported isotope effects suggests outer-sphere-single-electron transfer or S N 2 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.