Co-ordination of weak field ligands by N-acetylmicroperoxidase-8 (NAcMP8), a ferric haempeptide from cytochrome c, and the influence of the axial ligand on the reduction potential of complexes of NAcMP8

Abstract

N-Acetylmicroperoxidase-8 (NAcMP8), a ferric haempeptide derived from cytochrome c, retains the His-18 ligand and, in addition, has a readily-displaced coordinated water molecule. The co-ordination of ligands which leave Fe(III) in a predominantly high spin state has been investigated by difference UV–visible spectrophotometry. The affinity for these ligands is low (K = 0.69, 0.45, 0.46 and 0.35 dm³ mol⁻¹ for co-ordination of Cl⁻, Br⁻, I⁻ and SCN⁻, respectively), and somewhat larger for N₃⁻ (K = 25 dm³ mol⁻¹), where the metal is in an equilibrium between an admixed spin state (S = 3/2, 5/2) and a low spin state (S = 1/2). By contrast, previous work has shown that the affinity of Fe(III) for ligands that produce a low spin state is considerably larger (K = 10² to >10⁶ dm³ mol⁻¹). The effect of the axial ligand on the potential of the Fe(III)|Fe(II) couple in complexes of L–NAcMP8 (L = ligand trans to His-18) in aqueous solution at a glassy carbon electrode has been examined by cyclic voltammetry. The potentials of the quasi-reversible reduction of the halo complexes of NAcMP8 decrease in the order L = Cl⁻ > Br⁻ > I⁻ and reflect the hardness of the coordinated anion. The reduction potentials of some low spin complexes investigated (L = imidazole, ethanolamine, glycine, propylamine, cyanide) span less than 50 mV (E_½ = −0.203 ± 0.015 V vs. NHE), and are similar to complexes in which the metal is in a spin equilibrium (L = OH⁻, N₃⁻; E_½ = −0.209 ± 0.005 V), whereas high spin complexes (L = H₂O, Cl⁻, Br⁻, I⁻, SCN⁻) have somewhat higher reduction potentials (E_½ = −0.146 ± 0.023 V). Work with pyridine and primary amine ligands shows that an increase in ligand basicity stabilises the Fe(III) state relative to the Fe(II). The rate constants for heterogeneous electron transfer are relatively insensitive to the nature of the axial ligand. Thus, changing the axial ligand of an iron porphyrin modulates the reduction potential only by a relatively small amount. The implications of this observation for biological systems is discussed.