Correlations among parameters that describe low-spin ferric heme complexes

Abstract

The g values from low-spin ferric hemes can be related through the t 2 g hole model to rhombic ( V λ ) and tetragonal (Δ/λ) ligand field components and to the lowest Kramer's doublet energy ( E λ ). The latter is also a measure of unpaired electron sharing among the iron 3 d ( t 2 g ) orbitals. For a series of ligands (X), there is a monotonie increase in myoglobin complex (Mb · X) ¦ E λ ¦values with nonheme hexacoordinate metal complex (M · X 6) ¦ e g − t 2 g ¦orbital separations. As the aqueous solution p K a values of the sulfurous or nitrogenous ligands in model heme complexes increase, values of V λ and Δ/λ increase linearly, but those of ¦ E λ ¦decrease linearly. The greater the electron-acceptor ability of the ligand, as suggested by its position in the spectrochemical series or its p K a , the more the unpaired electron sharing among the heme t 2 g orbitals increases. The rate of change of ¦ E λ ¦with V λ and the p K a is different with sulfurous and nitrogenous ligands and the magnitude of both rates increases with two sulfurs < sulfur and nitrogen < two nitrogens bound to the heme. The maximum magnitude of this rate with V λ for cytochrome P-450 is four times less than that for myoglobin, which may explain, in part, the differences in ligand binding between these two hemeproteins. The perturbation of ¦ E λ ¦, V λ and Δ/λ induced by strain of iron-ligand bonds is quantitated for several hemeproteins and heme models. In addition, energy level comparisons suggest that the largest-magnitude g value falls approximately along the iron-chlorin ring normal. This suggestion implies that the electron distribution of the iron at the catalytic sites of cytochrome P-450 and certain chlorin-containing enzymes is in some way similar, but distinct from that at the transport site of myoglobin.