As a minimum, reactive systems for the reductive dehalogenation of haloorganic compounds consist of a source of reducing power, a chemical conduit for transferring electrons to the target compounds, and the halogenated organic compounds themselves. In biological systems, for reductive dehalogenation, reducing power is derived from organic substrates, and capable bacteria provide biochemical conduits for electron transfer to the target compounds. Abiotic systems can obtain reductant from a variety of sources including those investigated here - sunlight and an applied electrical potential. Such abiotic systems provide an excellent in vitro tool with which to test specific, biologically produced electron carriers as agents for the catalysis of electron transfer to haloorganic molecules. The practical importance of reductive processes for the initiation of destruction of the heavily halogenated organic compounds such as tetrachloroethylene and carbon tetrachloride arises from the relative recalcitrance of such highly armored and oxidized compounds to transformations under oxidizing conditions. We propose to investigate the mechanisms, kinetics, and limitations of reductive dehalogenation in three highly defined systems: (1) Under anaerobic conditions, Shewanella putrefaciens sp. 200, a facultative anaerobe, catalyzes the transformation of carbon tetrachloride to chloroform and as yet unidentified products. We will investigate the mechanisms of reductive dehalogenation in this system in whole-cell and cell-free experiments. Respiratory inhibitors will be used in conjunction with measurements of growth and adenosine triphosphate to determine whether or not the generation of biochemical energy is coupled to reductive dehalogenation in this system. (2) Semiconductor/macrocycle hybrid molecules will be used to convert radiant energy to reducing power in the form of conduction-band electrons and to transfer that reductant (via attached metallomacrocycles) to the compounds of interest. We will investigate the energetics, kinetics, mechanism, and limitations of this process. (3) In the photo-assisted electrolytic system to be investigated, reductant is provided voltammetrically to the cathodic compartment of an electrolytic cell. Biologically produced metalloenzymes will be used to lower overpotentials (catalyze electron transfer) associated with electron transfer to haloorganic compounds at the cathode, and light energy will be used to cleave metal-carbon bonds (release partially dehalogenated compounds) following their reduction. Proposed work is designed to promote a mechanistic understanding of reductive dehalogenation and may lead to commercially useful methods for destroying heavily halogenated organic compounds.
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