Oxo-ferryl porphyrin

Liu, M.H. and Y.O. Su (1998). Selective electrocatalysis of alkene oxidations in aqueous media. Electrochemical and spectral characterization of oxo-ferryl porphyrin, oxo-ferryl porphyrin radical cation and their reaction products with alkenes at room temperature. J. Electroanal. Chem. 452, 113-125. 

Perhaps the most important is the enzyme s ability to control the delivery of protons to the developing negative charge of the iron-dioxygen complex as electrons are introduced into the center. The protonation/deprotonation events at the bound superoxo, peroxo, and hydroperoxo anions were theoretically analyzed and claimed to account for the definitive reaction sequence in the formation of the active oxo-ferryl porphyrin... 

As briefly summarized in Section 1.2.2.1, extensive evidence has been reported indicating that horseradish peroxidase Compound I is an oxo-ferryl [Fe =0] porphyrin n-cation radical and that Compound II is an oxo-ferryl porphyrin. Groves and co-workers have reported an inorganic model complex for Compound I [53, 54] and Balch and co-workers have described a Compound II model [55, 56]. These models each appear to have the expected compositions for the respective enzyme states that they are designed to mimic. 

PREPARATION AND CHARACTERIZATION OF OXO-FERRYL PORPHYRIN CATION RADICALS BY MODEL SYSTEMS... 

On the basis of many spectroscopic measurements, the species was characterized to be an oxo-ferryl porphyrin K-cation radical (1), identical to compound I of peroxidases and catalases. Because of its high reactivity, introduction of sterically hindered groups at the ortho-positions of the phenyl rings are required to observe 1 as relatively stable species at low temperature. Alternatively, Nakamoto has reported the formation of compound I of Fe(OEP) and Fe(TPP) by laser irradiation to oxy forms of the complexes in oxygen matrices at 30 K [51, 52]. Because of the formation of compound I in the matrices, characterization of less sterically hindered porphyrin complexes can be made. 

Fujii has recently prepared oxo-ferryl porphyrin complexes of the Aiu type [59]. These complexes were reported to show comparable reactivity to those complexes having A2u-On the other hand, compound I models of Fe chlorine complexes, whose HOMO S are similar to Aiy, showed very poor reactivity in comparison with the corresponding porphyrin complexes [84]. The roles of HOMO of compound I are still subjects of future work. 

As described above, oxo-ferryl porphyrin 7c-cation complexes are two electrons oxidized from the resting ferric state. On the other hand, other types of two electron oxidized iron porphyrin complexes have been prepared. 

Scheme 3. The first example of oxo-ferryl porphyrin complex formation.

There have been a number of recent review articles on the application of EXAFS spectroscopy to the study of metalloproteins [1-8]. The theory of EXAFS spectroscopy and the historical development of the field have also been extensively discussed [1-5,7-19]. Here we will briefly cover the practical aspects of data analysis for biological heme (iron porphyrin) systems eind the appropriate model complexes. We will then focus on the EXAFS of two types of biological heme system (a) thiolate-ligated heme enzymes and (b) oxo-ferryl [oxo-iron (IV), Fe =0] states of heme enzymes. All of the enzymes discussed herein have in common the iron protoporphyrin IX ( heme ) as the prosthetic group, Fig. 1. 

Most of the model complexes for compound I are the derivatives of Fe(TPP) due to their stability. The HOMO s (highest occupied molecular orbital) of the porphyrin rings of these iron porphyrins are known to be A2u [73-75]. While Aiu and A2u characters of catalases and peroxidases are still controversial [53, 54, 76-82], as shown in Fig. 5, orbital pattern for the A2u and Aiu orbitals is very different [59, 83], thus, the reactivities of these two species are expected to be different. Especially that the pyrrole nitrogens of compound I having Aiu as HOMO is node indicates less interaction between the spin on the porphyrin ring and central oxo-ferryl orbitals. These considerations lead to the prediction that compound I whose HOMO is A2u is much reactive than Aiu lyP species. 

We have been considering that the active intermediate in most of the oxidation reactions catalyzed by P-450 is either oxo-ferryl (0=Fe(IV) porphyrin cation radical or its equivalent species, 0=Fe(V) complex. However, the last step of the metabolic transformation of androgens (6) to estrogens (7) by placental aromatase cytochrome P-450 (P-450arom) may require a different type of oxidant. The transformation of androgens by P-450arom is known to consist of three consecutive oxidations, each of which requires 1 mol of O2 and 1 mol of NADPH (Scheme 15) [256-258]. 

The oxidation of peroxidases by hydroperoxide leads to a ferryl iron-oxo species as well as a radical cation on the porphyrin ring, which is sometimes transferred to an adjacent amino acid. This species is referred to as compound I. Compound I can oxidize substrates directly by a two-electron process to regenerate the native peroxidase, but, more commonly, it oxidizes substrates by an one-electron process to form compound II where the porphyrin radical cation has been reduced. Compound II, in turn, can perform a second one-electron... 

---