FACTORS AFFECTING THE IRON OXYGEN VIBRATIONS OF FERROUS OXY AND FERRYL OXO HEME-PROTEINS AND MODEL COMPOUNDS

Resonance Raman (RR) and UV-visible absorption spectra of various imidazole-ligated ferrous oxy (FeII—O2) and ferryl oxo (FeIV=O) porphyrins are presented. Because physiologically relevant porphyrins have been used in this study, these measurements provide a basis to identify protein-induced changes in the iron–oxygen vibrations, as well as in the structure-sensitive macrocycle vibrations and in the optical absorption spectra, of heme enzyme intermediates. The iron-oxygen stretching frequencies, v(FeIV=O), of ferryl oxo complexes of octaethylporphyrin (OEP), protoporphyrin IX dimethyl ester (PPDME), and tetraphenylporphyrin (TPP) in toluene at -120 °C reported here, together with results from other model compounds, are used to interpret the values of v(FeIV=O) reported for ferryl-oxo myoglobin and the catalytic intermediates of several peroxidase enzymes. We identify the major protein influence on the v(FeIV=O) frequency to be the trans-ligand strength of the proximal histidine, with a minor, but nonetheless important, effect being imposed by distal hydrogen bonding of the oxo ligand (Sitter, A. J.; Reczek, C. M.; Terner, J. J. Biol Chem. 1985, 260, 7515-7522. Hashimoto, S.; Tatsuno, Y.; Kitagawa, T. Proc. Natl. Acad. Sci. U.S.A. 1986, 83, 2417-2421). This analysis follows from the inverse correlation we identify for v(FeIV=O) with respect to v(FeII=His). For the ferrous oxy complexes of OEP, protoporphyrin IX (PP), and porphyrin a (PA) in DMF and CH2C12 at -120 °C, the v(Fe,II=O2) values we obtain are compared with similar measurements on other model compounds. As opposed to the ferryl oxo systems, in which v(FeIV=O) correlates inversely with trans-ligand strength, a direct correlation exists between v(FeII=O2) and the strength of the iron/trans-ligand interaction in the model systems (Walters, M. A.; Spiro, T. G.; Suslick, K. S.; Collman, J. P. J. Am. Chem. Soc. 1980, 102, 6857-6858. Kerr, E. A.; Mackin, H. C.; Yu, N.-T. Biochemistry 1983, 22, 4373-4379). Moreover, we interpret the available data to indicate that v(FeIV—O2) correlates inversely with v(O=O) of bound dioxygen. We use these observations to evaluate the iron-oxygen vibrations of oxy myoglobin, horseradish peroxidase compound III, and an early intermediate in the reduction of O2 by cytochrome c oxidase; we conclude that the iron–oxygen stretching frequencies displayed by the various ferrous oxy proteins can be understood in terms of distal influences on the electron density in the O=O bond. © 1990, American Chemical Society. All rights reserved.