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

Abstract

Cysteine was studied as a reductant in carbon tetrachloride (CCl4) transformation mediated by vitamin B12 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 B12 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. B12r was the reduced B12 species but exhibited little reactivity toward CCl4 in the absence of cysteine; the reactive B12 species is hypothesized to be the pentacoordinated B12r−cysteinate complex. Most of the CCl4 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 B12 species involved in reductive biodehalogenation is likely to be either B12s or B12r−thiolate complexes.