Formation of porphyrin pi-cation radical in myoglobin. A study on one electron oxidation products of nickel (II)-substituted hemoproteins.

Nickel (II)-substituted myoglobin (Mb), hemoglobin (Hb) and horseradish peroxidase (HRP) were oxidized with iridate to examine whether porphyrin pi-cation radical is formed or not in these hemoproteins. It was found that Ni (II)-porphyrin pi-cation radical is formed in all of these hemoproteins as confirmed by UV-visible and ESR spectra, although the porphyrin pi-cation radical in Mb and Hb was less stable than in HRP. These results are discussed in relation to the different features of higher oxidation states of native Mb and HRP.