TRANSFORMATION OF CARBON-TETRACHLORIDE BY REDUCED VITAMIN-B-12 IN AQUEOUS CYSTEINE SOLUTION

Cysteine was studied as a reductant in carbon tetrachloride (CCl4) tra nsformation mediated by vitamin B-12 at room temperature in the pH ran ge of 4-14. The reaction proceeded in two consecutive phases: the init ial 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 alkylcoba lamin may control the rate in the slow phase. B-12r was the reduced B- 12 species but exhibited little reactivity toward CCl4 in the absence of cysteine; the reactive B-12 species is hypothesized to be the penta coordinated B-12r-cysteinate complex. Most of the CCl4 was transforme d to unidentified water-soluble products. The chloroform yield decreas ed 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.