Pc. Chiu et M. Reinhard, TRANSFORMATION OF CARBON-TETRACHLORIDE BY REDUCED VITAMIN-B-12 IN AQUEOUS CYSTEINE SOLUTION, Environmental science & technology, 30(6), 1996, pp. 1882-1889
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.