Transformation of Carbon Tetrachloride by Reduced Vitamin B₁₂ in Aqueous Cysteine Solution

Abstract

Cysteine was studied as a reductant in carbon tetrachloride (CCl 4 ) transformation mediated by vitamin B 12 at room temperature in the pH range of 4-14. The reaction proceeded in two consecutive phases: the initial phase was rapid but lasted only minutes before the slow subsequent phase started as B 12 was inactivated, presumably due to nonreactive alkylcobalamin formation. The reduction of Co(III) to Co-(II) was rate-limiting in the fast phase, whereas the decomposition of the alkylcobalamin may control the rate in the slow phase. B 12r was the reduced B 12 species but exhibited little reactivity toward CCl 4 in the absence of cysteine; the reactive B 12 species is hypothesized to be the pentacoordinated B 12r -cysteinate complex. Most of the CCl 4 was transformed to unidentified water-soluble products. The chloroform yield decreased with pH from 20% to nearly zero, whereas the carbon monoxide yield remained constant (3.2 ( 0.3%) with pH. These findings suggest that (1) the reductant controls both the kinetics and the mechanism of the reaction and should not be viewed simply as an electron donor, and (2) the B 12 species involved in reductive biodehalogenation is likely to be either B 12s or B 12r -thiolate complexes.

Biotransformation of carbon tetrachloride by various acetate- and nitrate-limited denitrifying consortia

“…The reductive dechlorination of CT to CF involves a two-electron-transfer hydrogenation reaction. The work of researchers studying the abiotic dechlorination of CT by cytochrome P-450 (Pohl et al, 1984) and vitamin B12 (Chiu and Reinhard 1996) strongly indicate this reduction occurs through the trichloromethyl radical ( ⅐ CCl 3 ). The trichloromethyl radical may then combine with a hydrogen atom to form CF, bind irreversibly with microsomal lipids, or be further enzymatically reduced to dichlorocarbene (:CCl 2 ) (Pohl et al, 1984).…”

The corrinoid cofactor of reductive dehalogenases affects dechlorination rates and extents in organohalide-respiring Dehalococcoides mccartyi

“…Intracellular corrinoids were extracted and purified following the KCN extraction protocol (Yan et al, 2013). Supernatant-associated corrinoids were reduced into the upper ligand-free, cobalt(II) form by reductants (for example, HS -, L-cysteine, dithiothreitol) present in the medium (Assaf-Anid et al, 1994;Chiu and Reinhard, 1996;Lesage et al, 1998). To convert cob(II)amides back to the oxidized cyanocob(III)amide forms, which can be separated and quantified with the established HPLC method, culture supernatants were reacted with ambient air in the presence of KCN for 24 h (Schneider and Stroiński, 1987).…”

Role of metalloporphyrin core metals in the mediated reductive dechlorination of tetrachloroethylene

“…A majority of the studies above utilized vitamin B 12 as the electron transfer mediator in their systems and have variously ascribed to it what we believe to be an overly simplified picture of the macrocycle's structure as illustrated in Figure 1a and b [1,11,12,21,29]. In both examples, the chemical structure shown is simplistic and fails to fully portray the complex threedimensional space description of the macromolecule.…”