METALLOCOENZYME-MEDIATED REDUCTIVE TRANSFORMATION OF CARBON-TETRACHLORIDE IN TITANIUM(III) CITRATE AQUEOUS-SOLUTION

Transformation pathways for carbon tetrachloride (CCl4) catalyzed by hematin or vitamin B-12 in aqueous titanium(III) citrate solution are proposed. The reaction of CCl4 with B-12 was zero Order in CCl4 and first order in B-12, and the rate constant was measured from pH 7.3 to pH 10.3. The proposed rate-limiting step is the reduction of the stable trichloromethylcobalamin (CCl3-Cbl) intermediate by titanium(III) citrate at alkaline pH and the sterically induced CCl3-Cbl decomposition at neutral pH. The reaction kinetics can be described by a modified Michaelis-Menten model in the saturated regime. With hematin, only the pseudo-first-order rate constant was determined due to the significant deactivation of the coenzyme. The turnover number of hematin (molecules of CCl4 transformed/molecule of hematin deactivated) was 27 at pH 8.0 and 42 at pH 9.9. Vitamin B-12 was a more stable and more effective catalyst (on a molar basis) than hematin with respect to CCl4. Chloroform (CHCl3) was the primary product in titanium(III) citrate solution, and the yield was a function of pH, Ti(III) concentration, and organic content regardless of whether a coenzyme was present or which coenzyme was used. Although B-12 and hematin can both enhance the CCl4 transformation rate, they have little effect on the CHCl3 yield. Titanium(III) citrate, on the other hand, controls not only the transformation rate but also CHCl3 formation.