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

N-Acetylmicroperoxidase-8 (NAcMP8), a ferric haempeptide derived from cytoc hrome c, retains the His-18 ligand and, in addition, has a readily-displace d coordinated water molecule. The co-ordination of ligands which leave Fe(I II) 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 dm(3) mol(-1) for co-ordination of Cl-, Br-, I- and SCN-, respectively), and somewhat larger for N-3(-) (K = 25 dm(3) 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 sho wn that the affinity of Fe(III) for ligands that produce a low spin state i s considerably larger (K = 10(2) to > 10(6) dm(3) mol(-1)). The effect of t he 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 ca rbon electrode has been examined by cyclic voltammetry. The potentials of t he quasi-reversible reduction of the halo complexes of NAcMP8 decrease in t he order L = Cl- > Br- > I- and reflect the hardness of the coordinated ani on. The reduction potentials of some low spin complexes investigated (L = i midazole, ethanolamine, glycine, propylamine, cyanide) span less than 50 mV (E-1/2 = -0.203 +/- 0.015 V vs. NHE), and are similar to complexes in whic h the metal is in a spin equilibrium (L = OH-, N-3(-); E-1/2 = -0.209 +/- 0 .005 V), whereas high spin complexes (L = H2O, Cl-, Br-, I-, SCN-) have som ewhat higher reduction potentials (E-1/2 = -0.146 +/- 0.023 V). Work with p yridine and primary amine ligands shows that an increase in ligand basicity stabilises the Fe(III) state relative to the Fe(II). The rate constants fo r heterogeneous electron transfer are relatively insensitive to the nature of the axial ligand. Thus, changing the axial ligand of an iron porphyrin m odulates the reduction potential only by a relatively small amount. The imp lications of this observation for biological systems is discussed.