Investigation of the catalytic cycle of the molybdoenzyme sulfite oxidase: Synthesis and spectroscopic study of model systems.
AuthorLaBarre, Michael James.
Committee ChairEnemark, John H.
MetadataShow full item record
PublisherThe University of Arizona.
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AbstractThis research has been directed at the study of the catalytic cycle of the molybdoenzyme, sulfite oxidase, through the use of both functional and structural model chemistry. A biologically relevant synthetic model for the first two steps of the proposed catalytic cycle of sulfite oxidase has been developed. A rapid oxygen atom transfer reaction from a dioxo-molybdenum(VI) center to triphenylphosphine is followed by an intermolecular electron/halogen exchange reaction between tetratolylporphinatoiron(III)-chloride and the now reduced oxo-molybdenum(IV) center. The kinetic and thermo-dynamic parameters of these reactions in dimethylformamide and toluene have been investigated and a self consistent mechanism has been proposed. Structural models for intramolecular electron transfer between the oxo-molybdenum center of the cofactor and the iron heme of sulfite oxidase have been prepared. Modified tetratolylporphyrins have been designed in order to contain a chelating catecholate functionality at discreet distances from the central cavity of the porphyrin ring. An oxo-molybdenum(V) group (which is stabilized by the facially coordinating, hydrotris(3,5-dimethyl-1-pyrazolyl)borate ligand) has been attached to the porphyrin through this catecholate functionality and the resulting bimetallic compounds have been investigated by NMR, EPR, UV/Vis, and electrochemical methods. The use of ³¹P-NMR spectroscopy as a probe of molybdenum-phosphate interactions in sulfite oxidase has been investigated through the synthesis and spectroscopic investigation of a series of six mononuclear oxo-molybdenum(V) and dioxo-molybdenum(VI) compounds which contain pendant phosphate esters. The ³¹P-NMR spectra of the Mo(V) compounds exhibit line broadening due to the absolute distance to the paramagnetic d¹ Mo(V) center. The relaxation times (T₁ and T₂) of the ³¹P center have been determined and are sensitive to the overall structure of the model compounds. The Mo-P distances have been calculated using the relaxation data and the Solomon equation and yield distances which are in reasonable agreement with the structures as determined by computer molecular modeling.