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

The EPR and magnetic Mossbauer spectra of a series of axial ligand complexes of tetrakis(2,6-dimethoxyphenyl)porphyrinatoiron(III), [(2,6-(OMe)2)4TPPFeL2]+, 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 "large gmax" EPR spectra with g>3.2 and well-resolved, widely spread magnetic Mossbauer spectra having A zz /gNμN>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 /gNμN 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 III 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 /gNμN 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 xx /gNμN=−565 kG, A yy /gNμN=629 kG, and A zz /gNμN=4 kG. A subset of Type II complexes (Type II′) have rhombicities (V/Δ) much greater than 0.67 and A zz /gNμN 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/Δ for Types I and II (but not for A zz versus V/Δ for Type II′). Not enough data are yet available on Type III complexes to determine what, if any, correlations may be observed.