PORPHYRIN CORE EXPANSION AND DOMING IN HEME-PROTEINS - NEW EVIDENCE FROM RESONANCE RAMAN-SPECTRA OF 6-COORDINATE HIGH-SPIN IRON(III) HEMES

Resonance Raman spectra are reported for high-spin bis(dimethyl sulfoxide) (Me2SO)2FeIII complexes of protoporphyrin IX dimethyl ester (PP), octaethylporphyrin (OEP), and tetraphenylporphyrin (TPP), and for aquo- and fluoromethem-oglobin (Hb) and myoglobin (Mb), with emphasis on the three bands (II, IV, and V) which have previously been shown to be sensitive to iron spin state. These frequencies are essentially the same for [(Me2SO)2FeIIIPP]+ as for aquo-metHb and Mb, which had previously been thought to be anomalously low. Since recent crystal structure determinations have shown the high-spin bisaquo and bis(tetramethylene sulfoxide) complexes of FemTPP to be planar porphyrins, with expanded cores, this experiment strongly supports core expansion as the determinant of the metHb and Mb spin-marker frequencies, and renders the hypothesis of protein-induced doming unlikely. Since protein crystallography has placed the iron atom 0.40 Å from the mean heme plane in metMb and 0.23 and 0.07 Å from the mean heme plane in the β and α chains of metHb, the lack of any associated variation in the spin-marker frequencies indicates that the core size is not determined by the disposition of the iron atom, but probably by steric interactions of the axial ligands with the pyrrole nitrogen atoms. Binding of fluoride to metHb and Mb leaves bands II and V unshifted, but lowers band IV by 5 cm-1. The similarity in spin-marker frequencies between native horseradish peroxidase (HRP) or cytochrome ć and the previously studied five-coordinate FeIII hemes indicates that these proteins contain five-rather than six-coordinate FeIII hemes; in the case of HRP six coordination is accessible with exogenous ligands. It is suggested that the intermediate-spin state observed for these proteins results from weakening of the bond between FeIII and the single axial ligand. The correlation with core size of the spin-marker frequencies is reexamined. Nonplanar hemes are found to have frequencies that are significantly depressed, presumably due to loss of π conjugation at the methine bridge. The relation between pyrrole tilt and the methine dihedral angles is derived for both ruffled and domed hemes, and is used to develop a simple relation that satisfactorily reproduces the spin-marker frequencies for both planar and nonplanar hemes. This model gives reasonable estimates of the σ and π contributions to the methine bond-stretching force constant. It is concluded that both core expansion and pyrrole tilting contribute to frequency lowerings, with core expansion dominant for moderate tilt angles. There is no evidence in any of the heme proteins so far studied of extra doming induced by the protein. © 1979, American Chemical Society. All rights reserved.