Resonance Raman studies of heme enzyme intermediates and metallocorroles
Abstract
Heme proteins are an important group of proteins and enzymes that possess an iron porphyrin derivative (i.e., heme) as a prosthetic group. In the enzymatic cycles of many of these heme enzymes, including cytochromes P450, heme peroxidases, catalyses and nitric oxide synthases (NO), several intermediates known as "peroxo", "hydroperoxo" and "compound-I" (i.e., oxoferrylporphyrin cation radical) have been proposed to arise by the reduction of oxygenated heme species followed by protonation of the distal oxygen atom and fast heterolytic scission of oxygen-oxygen bond. Although it was concluded from previous UV-Vis and EPR studies that irradiation of the oxygenated species of heme proteins with γ rays generates "peroxo" and "hydroperoxo" intermediates, this has never been observed by resonance Raman (RR) spectroscopy, which can serve as a direct probe of the iron-oxygen ligand fragments at the heme active site. In this work, RR spectroscopy was used for the first time to characterize these important peroxo/hydroperoxo intermediates. Metallocorroles are complexes that are structurally related to the well-known metalloporphyrins, which are important complexes in biology and catalysis. The recent interest in the metallocorroles was fuelled partly by the newly reported facile synthesis involving co-condensation of readily available precursors. It has been reported that metallocorroles, like their porphyrin analogues, are very efficient catalysts in atom (oxygen) and group (carbene, nitirine) transfer to organic substrates. Although there have been some reports on the vibrational (RR and infrared, IR) studies of metallocorroles, the lack of a sound interpretational framework precluded an in depth analysis of spectra/structure relations for these important complexes. In this work, RR and IR spectroscopies, in combination with Density Functional Theory (DFT) calculations, were used to study cobalt(III)-corrole and its isotopically labeled analogues in order to clarify the nature of the normal modes of vibration, and thus establish a reliable interpretational framework for future spectroscopic studies of these potential catalysts.
This paper has been withdrawn.