Resonance Raman Spectra of Dioxygen Adducts of Cobalt and Iron Porphyrins

Resonance Raman (RR) spectroscopy is potentially one of the most effective methods to investigate the structure and bonding of exogenous ligands of heme proteins and their model compounds. In fact, this technique can provide a direct probe of the metal-oxygen bond. Thus, the explicit goals of these studies are to identify the spectral features associated with the ν(O-O) (v: stretching), v(M-O2), and δ(M-O-O) (δ: bending) modes and to define the influence of steric, electronic and environmental factors on the corresponding vibrational frequencies. Although a good deal of progress has been made recently, full realization of these goals has yet to be attained. This work has been undertaken to provide better understanding of vibrational spectral patterns of cobalt and iron porphyrins and related compounds which react with molecular oxygen under various physicochemical conditions. The first part of the dissertation summarizes some of the physicochemical data of hemoglobins, myoglobins, cytochromes P-450, and horseradish peroxidases and their iron and cobalt model compounds which are essential to the understanding of the changes in structure and bonding during oxygen uptake. The second part is concerned with the investigation of vibrational coupling which occurs in RR spectra of the dioxygen adducts of cobalt porphyrins. Evidence of vibrational coupling of the v(O-O) of bound dioxygen with the associated solvent or solute molecules and also with the axial nitrogenous base ligand coordinated in the trans position to dioxygen is presented and discussed. This study has been undertaken to provide better understanding of the origin of the oxygen-sensitive multiple bands observed in the infrared (IR) spectra of oxyhemoglobin and RR spectra of cobalt substituted oxyhemoglobin. The third part of this dissertation describes the RR investigation and spectroscopic characterization of ferryl complexes as model compounds of cytochrome P-450 and horseradish peroxidase compound II. There is still a lack of full spectroscopic information about the active site of ferryl intermediate species which appear during catalytic cycles of these enzymes. Investigation of proper model compounds is essential in order to understand the structure and bonding of oxoiron(IV) compounds