Resonance Raman spectra of highly oxidized metalloporphyrins and heme proteins

Abstract

Studies of resonance Raman (RR) spectra of highly oxidized metalloporphyrins and heme proteins in the past decade are surveyed comprehensively. Following the introduction this article consists of two main sections. In Section 2 the Fe IV=O stretching ( v(Fe IV=O)) vibrations of ferryloxo neutral and /Gv cation radical porphyrins and their porphyrin in-plane modes are discussed and the characters of the Fe IV=O bond and environmental effects on it are elucidated. For porphyrin π cation radicals, the RR spectral differences between the a 1u and a 2u radicals are interpreted in relation to the electronic properties of those orbitals for divalent metalloporphyrins as well as iron porphyrins. Some changes in vibrational characters on oxidation of the porphyrin ring are noted on the basis of isotopic substitution data. Studies of environmental effects on porphyrin π cation radicals permit one to deduce factors for determination of radical types. Current RR studies of nitrido iron and N-oxide iron porphyrins and other metal oxo porphyrins with M IV, M V and M VI ions are also covered. In Section 3 the present state of RR studies of reaction intermediates of heme enzymes containing ferryloxo neutral and π cation radical porphyrins is summarized, and discussion is focused on the v(Fe IV=O) RR bands. Attention is paid to compound I of horseradish peroxidase for which complete historical vicissitudes of observed RR spectra are pursued. The most recent results from RR studies of reaction intermediates of cytochrome c oxidase with the Fe III-O-O-H and Fe IV=O hemes are explained in detail. Finally RR spectra of reaction intermediates of iron-chlorin containing enzymes as well as those of catalase and thiolate-ligated heme enzymes are reviewed.