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°

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 11, 111, 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 Mossbauer spectroscopy. The six complexes and the dihedral angles between axial ligand planes of each are [(TMP)Fe(1-Melm)(2)]ClO4 (0 degrees), paral-[(OMTPP)Fe(1-Melm)(2)]Cl (19.5 degrees), paral-[(TMP)Fe(5-MeHlm)(2)]ClO4 (26 degrees, 30 degrees for two molecules in the unit cell whose EPR spectra overlap), [(OETPP)Fe(4-Me2NPY)2]Cl (700), perp[(OETPP)Fe(1-Melm)(2)]Cl (731), and perp-[(OMTPP)Fe(1-Melm)(2)]Cl (900). Of these, the first three have been shown to exhibit normal rhombic EPR spectra, each 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,, being considerably smaller for the "parallel" complexes (400-540 kG) than for the strictly perpendicular complex (902 kG), A., 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-Melm)(2)]Cl, A, = 400 kG, A(xx) = -400 kG). In all cases, A,(zz)is small but difficult to estimate with accuracy. With results for six structurally characterized model systems, we find for the first time qualitative correlations of g(zz), A(zz), and Delta E-Q with axial ligand plane dihedral angle Delta(phi).