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

Transformation pathways for carbon tetrachloride (CCl4) catalyzed by h ematin or vitamin B-12 in aqueous titanium(III) citrate solution are p roposed. The reaction of CCl4 with B-12 was zero Order in CCl4 and fir st 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) citr ate at alkaline pH and the sterically induced CCl3-Cbl decomposition a t neutral pH. The reaction kinetics can be described by a modified Mic haelis-Menten model in the saturated regime. With hematin, only the ps eudo-first-order rate constant was determined due to the significant d eactivation 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 c atalyst (on a molar basis) than hematin with respect to CCl4. Chlorofo rm (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 coenzym e was used. Although B-12 and hematin can both enhance the CCl4 transf ormation rate, they have little effect on the CHCl3 yield. Titanium(II I) citrate, on the other hand, controls not only the transformation ra te but also CHCl3 formation.