COMPARATIVE PRODUCT ANALYSIS OF CARBON-TETRACHLORIDE DEHALOGENATION CATALYZED BY COBALT CORRINS IN THE PRESENCE OF THIOL OR TITANIUM(III) REDUCING AGENTS

Pathways of dehalogenation catalyzed by reduced Co corrins in the pres ence of various Co reductants are not well understood. Dehalogenation of CCl4 was determined in reaction systems containing a corrin (vitami n B-12, cobinamide dicyanide, or aquocobalamin), reductant [Ti(III), d ithiothreitol, or S-2-/cysteine], buffer (pH 8.2), and CCl4. Transform ation products in both the headspace and soluble reaction mixture were determined using GC/MS, HPLC, NMR, or TLC/autoradiography. Products i n the presence of titanium-(III) citrate, which yields the Co(I) form of the corrin, were mostly hydrogenolytic and included predominantly C H3Cl and CH4. More highly halogenated products were formed in the pres ence of thiol reducing agents, which yield the Co(II) form of the corr in. These products included CH2Cl2, CO, and formate in the case of dit hiothreitol. Sulfide/cysteine reductants were least effective in promo ting reductive dehalogenation with the major products identified as CH Cl3, CS2, 2-oxothiazolidine carboxylic acid, and 2-thioxo-4-thiazolidi necarboxylic acid. Unidentified water-soluble products were produced i n all assays with highest concentrations found in the presence of cobi namide dicyanide and thiol reductants. A proposed pathway includes the formation of a trichloromethyl radical as the first intermediate. Thi s radical forms an adduct with the reductant and, in the case of S-2-/ cysteine, ultimately produces CS2 or thiazolidines by way of thiophosg ene. Alternatively, the radical can be further reduced to form CHCl3 a nd CH2Cl2 or CO and formate by way of a dichloro carbene intermediate. Conjugation of thiol reductants with reactive intermediates is potent ially important in remediation and in predicting abiotic dehalogenatio n pathways in reduced environments.