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

Abstract

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 ( 13 CUNSWDHB =-4.3 ± 0.45‰;  13 CCF =-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 ( 13 CCF =-27.9 ± 1.7‰;  37 ClCF =-4.2 ± 0.2‰). In contrast, small carbon and unprecedented inverse chlorine isotope effects were observed for strain UNSWDHB ( 13 CUNSWDHB =-3.1 ± 0.5‰;  37 ClUNSWDHB = 2.5 ± 0.3‰) leading to opposing dual element isotope slopes (λCF = 6.64 ± 0.14 vs. λUNSWDHB =-1.20 ± 0.18). Isotope effects of strain CF were identical to experiments with TCM and Vitamin B12 ( 13 CVitamin B12 =-26.0 ± 0.9‰,  37 ClVitamin B12 =-4.0 ± 0.2‰, λVitamin B12 = 6.46 ± 0.20). Comparison to previously reported isotope effects suggests outer-sphere-singleelectron transfer or SN2 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. Non-directed 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.

Use of dual element isotope analysis and microcosm studies to determine the origin and potential anaerobic biodegradation of dichloromethane in two multi-contaminated aquifers

“…Several laboratory studies have applied dual C-Cl isotope analysis to describe biotic and abiotic transformation mechanisms for DCM (Chen et al, 2018;Heraty et al, 1999;Torgonskaya et al, 2019) and CF (Heckel et al, 2019(Heckel et al, , 2017aRodríguez-Fernández et al, 2018a;Torrentó et al, 2017) but, to the best of the author's knowledge, the application of a dual isotope approach in DCM-contaminated sites has not been reported yet.…”

Partial Hessian vibrational analysis: vapour pressure isotope effects and their relation with non-covalent interactions

“…3 by passive evaporation. 4 Considering position-specific isotope effects smaller than one. 5 Considering position-specific isotope effects larger than one.…”

“…Equilibrium isotope effects (EIEs) are one of the most successful tools in biochemistry, geochemistry, and chemistry for meticulously studying exchange reactions during processes such as volatilisation, sorption, and evaporation. [1][2][3][4] Given processes are vital to comprehend pollution, biodegradation pathways and remediation of contaminated aquiferous systems like oceans, lakes, and underground reservoirs. Halogenated compounds such as trichloromethane and bromobenzene represent a global concern, given that are not merely perilous pollutants with suspected carcinogen properties, but equally are known for their slow dissolution (per years) in anoxic environments.…”

“…Halogenated compounds such as trichloromethane and bromobenzene represent a global concern, given that are not merely perilous pollutants with suspected carcinogen properties, but equally are known for their slow dissolution (per years) in anoxic environments. 4 Technological advances during the last two decades permit now the accurate measuring of position specific isotope effects, hence providing an valuable insight into the nature of bulk isotope effects. However, precise interpretation of experimental results remains a complex task, predominantly owing to physicochemical phenomena that might be easily overlooked such as diffusion or adsorption, but posses a substantial influence over the outcome.…”

Mechanistic Aspects Regarding the Ultraviolet Degradation of Polychlorinated Biphenyls in Different Media: Insights from Carbon and Chlorine Isotope Fractionation

“…These additional effects could lead to controversial interpretation of the experimental data with only single-element CSIA, as the isotope fractionation factor could be altered not only by the different mechanisms but also by these additional effects. It is worth noting that these additional effects usually synchronously change different elements and are almost counteracted with dual-element isotope plots (Λ), − that is, graphs in which changes in isotope values of one element are plotted against those of a second. Hence, dual/multielement isotope analysis could provide a more reliable assessment about the underlying reaction mechanisms.…”