Myoglobin as a model for nitrite reductase reactivity and iron(TPP) catalyzed oxygen-atom transfer reaction

摘要

The project utilizes the surfactant film methodology developed by Rusling to examine myoglobin electrochemistry, as well as catalysis. When contained inside surfactant films, myoglobin exhibits enhanced electrochemical response. Careful examination of the initial cyclic voltammograms reveals a dynamic process between two different species. Similar behavior can be seen in tetrazolyl myoglobin, but not in the presence of strong binding ligands, such as azide and cyanide. A square scheme is proposed which describes equilibration between these two different redox active forms of the protein.;Further, myoglobin in surfactant films can be readily reduced to Fe(I) state, which proves to be a powerful catalyst for the reduction of nitrite. Two new waves are found in cyclic voltammograms, corresponding to the different stage of this multi-electron, multi-proton process. Both pH and nitrite concentrations affect the response, and a Kcat of 1.6 mM is obtained, similar to the nitrite binding constant. A variety of products are generated from bulk electrolysis, both in aqueous and gaseous phase. Both nitric oxide and hydroxylamine increase the catalytic efficiency, while the presence of ammonia slows down the reaction, typical of enzymatic reactions.;Myoglobin is also found to reduce nitric oxide catalytically. A stable intermediate, namely nitroxyl myoglobin, is observed at high pH solution. Single-turn over experiment on nitrosyl myoglobin confirms our hypothesis, and a lifetime of seconds is obtained. This species reacts with free nitric oxide in solution instantaneously, as calculated from digital simulation of our voltammograms.;The reduction of nitrosyl myoglobin in solution is achieved utilizing a Cr(II) reducing reagent. Proton NMR shows that even at pH as high as 10, this nitroxyl species is protonated. The relative spatial arrangement is obtained from 2D NOESY experiment. This is the first report of such a complex in aqueous solution.;The second part of the research entails the mechanistic study of nitric oxide disproportionation reaction catalyzed by Fe(TPP). Kinetic evidence obtained from solution phase UV-Vis spectra and solid phase IR suggests the presence an N-N coupled intermediate. The species is found to transfer an O-atom to PPh3, thus uncouples the formation of nitrite from the production of N2O. Transient IR spectra reveals two new isotopic sensitive peaks at 1898 and 1858 cm-1, close to the free cis-(NO)2 dimer. To our knowledge, this is the first characterization of such species in a discrete metal complex.