Models of the Membrane-Bound Cytochromes:  Mössbauer Spectra of Crystalline Low-Spin Ferriheme Complexes Having Axial Ligand Plane Dihedral Angles Ranging from 0° to 90°

Abstract

Crystalline samples of four low-spin Fe(III) octaalkyltetraphenylporphyrinate and two low-spin Fe (III) tetramesitylporphyrinate complexes, all of which are models of the bis-histidine-coordinated cytochromes of mitochondrial complexes II, III and IV, and chloroplast complex b 6 f, and whose molecular structures and EPR spectra have been reported previously, have been investigated in detail by Mössbauer spectroscopy. Of these, the first three have been shown to exhibit normal rhombic EPR spectra with three clearly-resolved g-values, while the last three have been shown to exhibit "large g max " EPR spectra at 4.2 K. It is found that the hyperfine coupling constants of the complexes are consistent with those reported previously for low-spin ferriheme systems, with the largest-magnitude hyperfine coupling constant, A zz , being considerably smaller for the "parallel" complexes (400-540 kG) than for the strictly perpendicular complex (902 kG), A xx being negative for all six complexes, and A zz and A xx being of similar magnitude for the "parallel" complexes (for example, for [(TMP)Fe(1-MeIm) 2 ]Cl, A zz = 400 kG, A xx = -400 kG), and finally, A yy is small, but difficult to estimate with accuracy for all complexes. With results for six structurally-characterized model systems we find qualitative correlations of g zz , A zz , and △E Q with axial ligand plane dihedral angle △φ.

Review: Studies of ferric heme proteins with highly anisotropic/highly axial low spin (S = 1/2) electron paramagnetic resonance signals with bis‐Histidine and histidine‐methionine axial iron coordination

“…However, for His/Met ligated systems, no clear–cut links have been found between ligand-field parameters and axial groups’ orientations. In fact, looking at the NMR and EPR results shown in Table 3, there is no direct relationship between the presence of axial Met dynamics and observation of axial EPR spectra, nor between Met orientation (A, B) (Figure 8) and g –tensor parameters, in contrast with cytochromes with bis–His axial ligation for which ligand orientation and g –tensor parameters are clearly correlated 93,94. On the other hand, as shown in Figure 11, a linear correlation stands between the observed g max values (or their derived ligand–field anisotropy, V/Δ ratio) versus the averaged heme methyl chemical shift <δ> that can also accommodate the values taken from some other cytochromes c with His-Met axial ligation.…”

Modulation of the Ligand-Field Anisotropy in a Series of Ferric Low-Spin Cytochrome c Mutants derived from Pseudomonas aeruginosa Cytochrome c-551 and Nitrosomonas europaea Cytochrome c-552: A Nuclear Magnetic Resonance and Electron Paramagnetic Resonance Study

“…The two histidine ligands coordinating the heme iron in these systems are both orthogonal to the heme plane. When the two axial ligands lie in perpendicular planes the HALS EPR signal is observed, and if the axial ligands adopt a parallel orientation, the resulting EPR signal is much more rhombic 39,40. From these studies, correlation of structure with the observed electronic properties of the heme group shows that the presence of strong axial ligands (strong σ-donors and weak π -acceptors), a highly saddled-shaped heme plane, and/or electron withdrawing substituents at the meso positions of the porphyrin cause the heme configuration to be stabilized towards the axial (d xy ) 2 (d xz ,d yz ) 3 state.…”

Explorations in metalloporphyrin stereochemistry, physical properties and beyond

“…Very bulky ligands and the use of picket fence porphyrin gave complexes with relative perpendicular orientations in iron(II) species [62,66]. Additional iron(II) and iron(III) studies are detailed in references [61][62][63][64].…”