The project concerns important aspects of the chemistry of metalloporphyrin that serve as good models for hemoproteins such as hemoglobin, myoglobin, cytochromes, and peroxidases. These proteins function in a variety of processes, including oxygen transport and storage, electron transfer catalysis, peroxide utilization, and detoxification. There are several project goals. (1) We plan to investigate the influence of hydrogen-bonding from coordinated imidazole and propionic acid side chains on the heme redox potetials and the reactivity of coordinated ligands. (2) 59 Co NMR will be used to probe the electronic environment about the metal in cobalt(III) porphyrins with respect to ring substituent and axial ligand variation, and hydrogen-bonding to/from axial ligands. We shall attempt to define a correlation between 59C0 and 57Fe NMR chemical shifts so that 57Fe chemicals shifts can be predicted for iron porphyrins and, ultimately, hemoproteins. (3) Electrochemistry at ca. -90 degrees C in relatively nonpolar solvent (e.g., dichloromethane) will be applied to thermally unstable species that model intermediates in peroxidase and cytochrome P-450 mediated reactions. The compounds to be studied include prophyrins (TPP,PPIX, PPIXDME, chelated hems) with the following relevant links: M-O2(M = Fe,Co), Fe-O-O-Fe, Fe-O, and Co(SR)O2. The purpose here is to study the redox behavior of these species and relate this to the protein reactions. (4) The applicability of pulsed laser optoacoustic spectroscopy to problems in metalloporphyrin and other biochemical dynamics will be tested. This may allow reactions to be studied at far lower reactant concentractions that previously possible. A variety of techniques are to be utilized. The kinetics will be followed via stopped-flow methods at temperatures from +50 degrees C to -70 degrees C. Others will be followed by the optoacoustics methods; this has never before been attempted and may be highly significant for studying protein reactions because only ca. 10-8 M concentrations are required. The electrochemistry will entail cyclic voltammetry, coulometry, and various polarographic techniques. The most promising is our recently develped method for doing cyclic voltammetry at very low temperatures in unreactive solvents. This may be extended to nonpolar solvents (e.g., toluene) by using microelectrode technology. 59Co NMR will be done at various field strengths to ascertain the importance of chemical shift anisotropy on the chemical shifts and linewidths, which relate to the d-d excitation energies in the porphyrin complexes. The proposed experiments offer an opportunity to delineate several important aspects of hemoprotein models. These include (1) the role of hydrogen-bonding on reactivity and redox behavior, (2) the electron transfer chemistry of reaction intermdiates, and (3) the electronic influence of the prophyrin ligand in comparison to other donor molecules.
| Baer, C D; Stone, N J; Sweigart, D A (1988) Fabrication of platinum-disk ultramicroelectrodes. Anal Chem 60:188-91 |
