The coordination of imidazole and substituted pyridines by the hemeoctapeptide N-acetyl-ferromicroperoxidase-8 (Fe IINAcMP8)

Abstract

The N-terminus acetylated ferric hemeoctapeptide from cytochrome c, N-acetylmicroperoxidase-8 (Fe III-NAcMP8) can be reduced by dithionite in aqueous solution to produce Fe II-NAcMP8. The UV–Vis spectrum has a broad Soret band and relatively poorly defined Q bands which is consistent with a mixture of a five-coordinate high spin species with His as the axial ligand and a six-coordinate, predominantly high spin species with His/H 2O as axial ligands. There are two spectroscopically observable p K as at 8.7 ± 0.1 and 10.9 ± 0.2 which are attributed to ionization of a heme propionic acid group and coordinated H 2O, respectively; a p K a ⩾14 is due to ionization of the proximal His ligand. Equilibrium constants were determined by UV–Vis spectrophotometry at 25.0 ± 0.2 °C and 0.5 M ionic strength (NaClO 4) for the coordination of imidazole and a number of substituted pyridines, and complement available data for the ferric hemepeptide, allowing a comparison to be made of the affinity of an iron porphyrin with Fe in the +2 and +3 oxidation states towards these ligands. Imidazole is coordinated more strongly by the ferric porphyrin (log K = 4.08) than by the ferrous porphyrin (log K = 3.40). The equilibrium constants for coordination of pyridines by the ferric and ferrous porphyrins increase and decrease, respectively, with increasing ligand basicity. Values determined by cyclic voltammetry show the same dependence on the identity of the ligand. In the ferric porphyrin, the stability of the complex increases with the basicity of the ligand and hence its ability to donate electron density onto the metal. In the case of the more electron rich ferrous porphyrin, greater stability occurs with pyridine ligands that have an electron withdrawing group and hence can accept electron density from the metal. This is consistent with the midpoint reduction potentials E 1/2 of the pyridine complexes determined by cyclic voltammetry; E 1/2 is linearly dependent on, and becomes more negative with an increase in, ligand basicity. Log K for coordination of pyridines by the ferrous hemepeptide correlates well with the energy of the ligand frontier orbital with π symmetry, suggesting that π-bonding effects are significant in determining the strength of binding of pyridines by a ferrous porphyrin.