Methanotrophic oxidation of substituted aromatic compounds: A mechanistic approach to biodegradation and quantitative structure-activity relationships

Substituted biaryl compounds such as polychlorinated biphenyls are ubiquitous environmental contaminants resulting from agricultural and industrial applications. Whereas these molecules have been demonstrated to undergo transformation reactions catalyzed by aerobic heterotrophs and under reducing conditions in sediments and groundwater aquifers, ortho-substitution generally tends to confer recalcitrance to these molecules in both microbial systems.; This work was aimed at describing and quantifying the transformation potential and oxidation mechanisms that may play a role in the degradation of ortho-substituted biphenyls by mixed and pure cultures of microorganisms representative of the interface between both boundary conditions mentioned above. Methanotrophs, expressing the soluble form of the methane monooxygenase, both as pure and mixed methanotrophic/heterotrophic cultures, were used to evaluate the effect of a wide range of substituents and of microbial interactions on the kinetics of oxygen uptake and product distribution. Mechanistic inferences from the kinetic studies were obtained using quantitative structure-activity relationships (QSARs) composed of electronic (Hammett and quantum-mechanical), size, and hydrophobicity descriptors.; The transformation capacity of the methanotrophic cultures was tested against the ortho-substituted compounds including (multiple) ring hydroxylations, substituent oxidations, as well as substituent elimination. The electronic characteristics, size, and hydrophobicity of the substituent affected the rates and products of oxidation by each methanotroph differently. For example, oxidation by Methylosinus trichosporium OB3b was most affected by the electronic properties, while oxidation by strain CSC1 was evenly impacted by most molecular properties included. The types and abundances of products formed and the relatively high kinetic isotope effects observed were consistent with both direct and indirect oxidation mechanisms, which include both hydrogen abstraction and an intramolecular (NIH) shift of the substituent. The oxidation kinetics by mixed culture MM1 was higher than that of the derived methanotrophic strain CSC1, implying the positive interactions that may occur among these populations.; This integrated experimental-modeling approach has provided a means to qualitatively and quantitatively assess the potential contributions of methanotrophs to aryl oxidation and has indicated isolate-specific mechanistic differences as well as ecological implications of methanotrophic-heterotrophic interactions.