High-valent iron porphyrins

The term 'high-valent' refers to iron complexes of porphyrins and related m acrocycles in which the oxidation state of the iron center exceeds III. Hig h-valent iron porphyrins and chlorins are important biological transients w hose intermediacy has been demonstrated in numerous peroxidase and catalase enzymes. Two species, compounds I and II, are spectroscopically detectable upon stoichiometric addition of monooxygen donors to resting ferric enzyme s. Compounds I and II are formally two and one oxidizing equivalents respec tively above the ferric state. In compound II the oxidizing equivalent has been shown by spectroscopic studies to be located on iron as an oxoiron(IV) unit. The spectroscopic and magnetic properties of compound I support the structural assignment of an S=1 oxoiron(IV) unit magnetically coupled to a heme pi-cation radical (S = 1/2). Studies on model hemes have contributed m uch to the understanding of protein chemistry. Much work has been accomplis hed with meso-tetaarylporphyrins and, more recently, with physiologically c ongruent meso-unsubstituted pyrrole beta-substituted complexes. Compounds I of both proteins and synthetic models have been characterized by a wide ar ray of spectroscopic methods, including UV-vis, NMR, resonance Raman, EPR, variable-temperature/variable-field magnetic Mossbauer, magnetic circular d ichroism and extended X-ray absorption fine structure spectroscopy. Results of these studies are summarized. Recent developments, which promise to yie ld a detailed picture of electronic structure, are variable-temperature mag netic circular dichroism, studies in the pre-K-edge region and L-edge X-ray absorption spectroscopy. Time-resolved X-ray diffraction techniques have b een applied to obtain the first structural data on the protein forms of com pound I. Copyright (C) 2000 John Wiley & Sons, Ltd.