Models of the bis-histidine-coordinated ferricytochromes: Mossbauer and EPR spectroscopic studies of low-spin iron(III) tetrapyrroles of various electronic ground states and axial ligand orientations

摘要

The EPR and magnetic Mossbauer spectra of a series of axial ligand complexes of tetrakis(2,6-dimethoxyphenyl)porphyrinatoiron(III), [(2,6)-(OMe)_2)_4 TPPFeL_2]~+, where L = N-methylimidazole, 2-methylimidazole, or 4-(dimethylamino)pyridine, of one axial ligand complex of tetraphenylporphyrin, the bis(4-cyanopyridine) complex [TPPFe(4-CNPy)_2]~+ and of one axial ligand complex of tetraphenylchlorin, [TPCFe(ImH)_2]~+, where ImH-imidazole, have been investigated and compared to those of low-spin Fe(III) porphyrinates and ferriheme proteins reported in the literature. On the basis of this and previous complementary spectroscopic investigations, three types of complexes have been identified: those having (d_(xy))~2(d_(xz),d_(yz))~3 electronic ground states with axial ligands aligned in perpendicular planes (Type I), those having (d_(xy))~2(d_(xz),d_(yz))~3 electronic ground states with axial ligands aligned in parallel planes (Type II), and those having the novel (d_(xz),d_(yz))~4(d_(xy))~1 electronic ground state (Type III). A subset of the latter type, with planar axial ligands aligned parallel to each other or strong macrocycle asymmetry that yield rhombic EPR spectra, cannot be created using the porphyrinate ligand. Type I centers are characterized by "largeg_(max)" EPR spectra with g > 3.2 and well-resolved, widely spread magnetic Mossbauer spectra having A_(zz)/g_Nμ_N > 680 kG, with A_(xx) negative in sign but much smaller in magnitude than A_(zz), while Type II centers have well-resolved rhombic EPR spectra with g_(zz) = 2.4-3.1 and also less-resolved magnetic Mossbauer spectra, and usually have A_(zz)/g_Nμ_N in the range of 440-660 kG (but in certain cases as small as 180 kG) and A_(xx) again negative in sign but only somewhat smaller (but occasionally larger in magnitude) than A_(zz), and Type II centers have axial EPR spectra with g_⊥ ≈ 2.6 or smaller and g_‖ < 1.0-1.95, but often not resolved, and less-resolved magnetic Mossbauer spectra having A_(zz)/g_Nμ_N in the range of 270-400 kG, and A_(xx) again negative in sign but much smaller in magnitude than A_(zz). An exception to this rule is [TPPFe(4-CNPy)_2]~+, which has A_(zz)/g_Nμ_N = -565 kG, A_(zz)/g_Nμ_N = 629 kG, and A_(zz)/g_Nμ_N = 4 kG. A subset of Type II complexes (Type II') have rhombicities (V/△) much greater than 0.67 and A_(zz)/g_Nμ_N ranging from 320 to 170 kG, with A_(xx) also negative but with the magnitude of A_(xx) significantly larger than that of A_(zz). These classifications are also observed for a variety of ferriheme proteins, and they lead to linear correlations between A_(zz) and either A_(xx),g_(zz), or V/△ for Types I and II (but not for A_(zz) versus V/△ for Type II'). Not enough data are yet available on Type III complexes to determine what, if any, correlations may be observed.