Spectroscopic studies of ferric porphyrins with quantum mechanically admixed intermediate-spin states: models for cytochrome c'

Abstract

Several fiveand six-coordinate ferric octaalkylporphyrin (OEP) complexes [Fe(OEP)X, X = CIO,-, S03CF3-, SbFL; [Fe(OEP)(3,5-C12Py)Z]C104, Py = pyridine] previously reported to display quantum mechanically admixed intermediate-spin behavior have been spectroscopically examined as possible models for cytochrome c’. The magnetic behavior of the complexes has been confirmed using N M R and EPR spectroscopy. The magnetic circular dichroism (MCD) spectra of these complexes have been investigated in the Soret/visible (300-700 nm) and near-IR (700-2000 nm) regions for the first time and compared to the spectra of high-spin Fe(0EP)CI and low-spin [Fe(OEP)(HIm)2]CI and [Fe(OEP)(NMeIm)X Suzuki, S.; Nakahara, A.; Iwasaki, H.; Masuko, M.; Matsubara, T. Biochim. Biophys. Acta 1985,831, 267-274). The near-IR MCD spectra of these complexes are also very similar to those of cytochrome c’isolated from several bacterial sources (Rawlings, J.; Stephens, P. J.; Nafie, L. A,; Kamen, M. D. Biochemistry 1977, 16, 1725-1729) and are distinctly different from those of known highand low-spin ferric heme systems or of complexes containing a thermal mixture of highand low-spin states. These results provide new evidence for the utility of MCD spectroscopy, especially in the near-IR region, as a probe of spin state in ferric heme systems and support the use of the quantum mechanically admixed intermediate-spin ferric porphyrins examined herein as models of cytochrome c’.