Compounds I of Catalase and Horse Radish Peroxidase : 7 r - Cation Radicals

Two-electron oxidation of cobaltous actaethylporphyrin [Co(II)(Et).P] yields a stable ...... cation radical [Co(III)(Et)8P]'+·, the optical spectrum of which exhibits spectral changes dependent upon the nature of the counterion. COInparison of these spectra with those of Compounds I of horseradish peroxidase and catalase leads us to propose that these Compounds I contain a 7r-cation radical of the heme prosthetic group. This proposal explains the oxidation level, optical spectra, and stability of the priInary compounds without recourse to properties such as stoichiometric mixtures of special porphyrins, stable Fe(V) porphyrins, or unique conformers of heme porphyrins. Explanations are advanced to account for the missing electron spin resonance signal of Compound I of horseradish peroxidase. Recent work on the oxidation of metalloporphyrins has provided (1--4) examples in which one or two electrons are given up by the organic moiety to yield 7r-cations, as well as examples of oxidations in which one electron is abstracted from the metal and the other from the organic ring (4). The esr spectra of the 1I"-cation radical of zinc meso-tetraphenylporphyrin [Zn(U) (Ph). Pl+· CIO.-, and that of magnesiumoctaethylporphyrin [Mg(U)(Et)sPl+· CIO.-, clearly establish electron abstraction from the porphyrin ring. Comparison of the optical spectra of a number of such 1I"-cation radicals reveals that they fall in two categories typified by either Mg(Eth +. or Zn (Ph). + .. The Mg (Et)8 + . visible spectrum is characterized by a major absorption peak near 700 nm with a high-energy shoulder, while Zn(Ph).p+· exhibits a broad, nearly featureless absorption in the region 500-700 nm. These differences have been ascribed previously (4) to two close-lying ground states of the radicals: 2A1U (class 1) for Mg(Et)8P+·, and 2A2U (class 2) for Zn(Ph)4 + .. * We present here results on the chemical and electrochemical oxidation of Co(U)(Et)sP, which undergoes reversible metal and ring oxidations to yield [Co(UI) (Et)8P 1 + and [Co(UI) (Et)sP]H· . This radical possesses the additional feature that it exhibits class 1 and 2 spectra interchangeably as a function of the counter ion. Correlation of the stability and the optical properties of the metalloporphyrin cation radicals Abbreviations: HRP, horse radish peroxidase; (Ph)~, tetraphenylporphyrin; (Et)gP, octaethylporphyrin; sce, aqueous saturated calomel electrode. * Class 1 spectra are shown by [Mg(II)(Et)8P] +. ClO.and Br-; [Ni(II)(Ph)~] +. CIO, and CI-; [Zn(II)(Et)sP] +. CIO. and Br-j [Ag(II) (Et)8P] +. CIO.-: Class 2 spectra are shown by [Zn(II)(Ph)~] +. CIO,and Br-; [Co(III)(Ph).P]H·2CIO.-, 2Br-, and 2Cl-j [Cu(II)(Ph)~] +. CIO.[Mg(II)(Ph).P] +. CIO.-; [Cu(II)(Et)8P] +. CIO,and Br-; [Pd(II)(Et)8P] +. CIO.-; [Ni(II)(Et)8P] +. ClO.and CI(the last three examples are discussed in ref. 2). 614 with the known properties of CompOUl'l.ds I of catalase and peroxidase leads us to propose that these forms of the enzymes contain a 1I"-cation radical of the heme prosthetic group.