Resonance Raman study of ferryl porphyrins and their pi-cation radicals
Abstract
The essential goal of the present studies is to provide a spectroscopic characterization of highly-oxidized hemes and to document the environmental factors which dictate their reactivity. The principal technique used is Resonance Raman Spectroscopy. Metalloporphyrins serve as the prosthetic group in a number of proteins, which perform a variety of biological functions. Their diverse properties and functions are dictated by the structure and oxidation states of the prosthetic group in the resting state as well as in unique enzymatic intermediates. It has been proposed that the primary intermediate involved in enzymatic cycles of several oxidative heme enzymes are ferryl $\pi$-cation radicals which may assume either an $A\sb{1\rm{u}}$ or $A\sb{2{\rm u}}$ ground electronic state, depending on the relative energies of the two highest occupied molecular orbitals. The enzymatic systems are known to be very photosensitive and exposure to laser excitation during RR studies can lead to photoproduct. One advantage of studying the more simple model compounds is that such photochemistry can be reduced or eliminated, although this issue is currently also controversial. Thus, conflicting reports have appeared regarding the photosensitivity of the models. The present work is devoted to resolving this controversy regarding photosensitivity of the ferryl $\pi$-cation radicals. In addition, the effect of axial ligand binding and peripheral substituents on the nature of the heme $\pi$-cation radicals are documented. The first part of this dissertation summarizes some of the physicochemical data for oxidative heme enzymes, metalloporphyrins, ferryl porphyrins, and their porphyrin $\pi$-cation radicals. The effect of the nature of the trans-axial ligand on the RR spectrum of the ferryl porphyrin $\pi$-cation radical is investigated in the next part of these studies. Briefly, two distinct spectral patterns are observed in the resonance Raman spectra which are apparently dictated by the strength of the trans-ligand. The third part of this research is concerned with the photochemistry of the ferryl porphyrin $\pi$-cation radicals. Evidence for a photoreduction process of the ferryl porphyrin $\pi$-cation radical to the corresponding nonradical, ferryl and ferric porphyrin species is observed and discussed. A plausible mechanism, involving disproportionation of the photodegradated ferryl porphyrin in the presence of alcohol (methanol) is discussed in terms of the effects of concentration and the power of the excitation source. In the final chapter, the effect of peripheral substitution on the ground electronic state of the ferryl porphyrin $\pi$-cation radical is investigated using iron tetramethyltetramesityl porphyrin. The spectral features associated with the ferryl porphyrin $\pi$-cation radical and its neutral analogue are assigned and interpreted.
This paper has been withdrawn.