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

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.