Biodehalogenation: The kinetics and rates of the microbial cleavage of carbon-halogen bonds



Specific rate constants associated with defined molecular paths of carbon-halogen bond cleavage from a variety of alkyl halide substrates by six soil organisms are presented. Five aerobes (three pseudomonads, one methylotroph, and one flavobacterium) and one anaerobe (a methano-gen) are compared. The rate constants were obtained with resting cells in phosphate buffer at pH 7.4 in the absence of nutrients or other substances. The observed general rate law is d(X)/dt = k (RX)(organisms), wherein X = halide ion, RX = alkyl halide, and (organisms) = number of organisms per liter. At concentrations of substrates ≥10−3 M, the rate may diminish or cease due to the toxicity of the substrate. At high cell densities ≥0.1 g/ml wet weight, the rates may saturate. This is attributed to a packing phenomenon of the cells. There is no simple correlation of rates with the chemical structure of the substrates when the conversions are grouped by reaction type; however, the order of reductive hydrogenolysis roughly follows the carbon-halogen bond dissociation energies. Several factors may contribute to a modulation of the rates observed with defined enzymatic active site models in homogeneous solution. These factors are briefly discussed. The aerobes exhibited a greater chemical diversity than the anaerobe in transforming these substances, and they react with much greater speed. In these biotransformations reductive processes occur more often than substitutions or oxidations. A surprising result was that the aerobes can reductively dehaloge-nate more rapidly than the methanogen.