Ferryl species structure

A less common reactive species is the Fe peroxo anion expected from two-electron reduction of O2 at a hemoprotein iron atom (Fig. 14, structure A). Protonation of this intermediate would yield the Fe —OOH precursor (Fig. 14, structure B) of the ferryl species. However, it is now clear that the Fe peroxo anion can directly react as a nucleophile with highly electrophilic substrates such as aldehydes. Addition of the peroxo anion to the aldehyde, followed by homolytic scission of the dioxygen bond, is now accepted as the mechanism for the carbon-carbon bond cleavage reactions catalyzed by several cytochrome P450 enzymes, including aromatase, lanosterol 14-demethylase, and sterol 17-lyase (133). A similar nucleophilic addition of the Fe peroxo anion to a carbon-nitrogen double bond has been invoked in the mechanism of the nitric oxide synthases (133). 

In view of the formation of a highly reactive Compound I ferryl species, and the fact that the porphyrin radical cation of this intermediate is reduced in enzymes such as CcP by a protein residue, it is not surprising that permanent covalent modifications are autocatalytically introduced into some protein frameworks. Two examples of autocatalytic protein modification, those of LiP and the catalase-peroxidases, are summarized here to illustrate the maturation of peroxidase protein structures that can have important functional consequences. 

There are now good theoretical descriptions of the electronic structures contributing to the optical absorption bands in spectra of porphyrin radicals and ferryl species [160,167] most charge-transfer bands in the latter are due to a transition from a porphyrin p orbital to an Fe-0 tt orbital [167], However, in the absence of a prior knowledge of the structure around the Felv site (and/or spectra of a variety of synthetic model compounds) it is not straightforward to assign an optical spectrum to a ferryl species. Thus the intermediate assumed to be the ferryl species in the binuclear haem c /Cub centre of cytochrome c oxidase [168] has a spectrum at 580 nm essentially identical [169] to that of low-spin ferric haem a3 compounds (e.g. cyanide). 

The reported experimental data, DFT-derived structural electronic and energetic information, and mechanistic interpretations, lead to a consistent picture, which is in fine with observations with other ligand systems, although there is some disagreement in terms of possible interpretations. However, there are a number of open questions (e.g., regarding the spin state of the ferryl species, which for both the tetra- and pentadentate ligand-based systems is predicted by DFT to be close to the spin cross-over limit) (175). Therefore, some of the current interpretations might be revised in the future. 

A conceptual model has been proposed (Fig. 53), in which the redox potential of the Fe + center was modified to suppress the irreversible conversion to the Fe +—O—Fe + /jl-oko complex in favor of the /u.-peroxo species Fe " "—O—O—Fe " ". It has been suggested that via this route the formation of the ferryl species, 0=Fe, would be facilitated. Four catalysts have been prepared on these principles iron perhaloporphyrins complexes Keggin structures with iron in the framework and with proximate iron centers and crystalline iron zeolite materials. 

N-substituted iron porphyrins form upon treatment of heme enzymes with many xenobiotics. The formation of these modified hemes is directly related to the mechanism of their enzymatic reactivity. N-alkyl porphyrins may be formed from organometallic iron porphyrin complexes, PFe-R (a-alkyl, o-aryl) or PFe = CR2 (carbene). They are also formed via a branching in the reaction path used in the epoxidation of alkenes. Biomimetic N-alkyl porphyrins are competent catalysts for the epoxidation of olefins, and it has been shown that iron N-alkylporphyrins can form highly oxidized species such as an iron(IV) ferryl, (N-R P)Fe v=0, and porphyrin ir-radicals at the iron(III) or iron(IV) level of metal oxidation. The N-alkylation reaction has been used as a low resolution probe of heme protein active site structure. Modified porphyrins may be used as synthetic catalysts and as models for nonheme and noniron metalloenzymes. 

The same ferryl porphyrin radical, [(P -) (Fe =0)]+, electronic structure is believed to exist for the activated oxygen-inserting species of cytochromes P450, although attempts to trap this species by freeze-quench experiments utilizing peroxyacetic acid on the timescale of Sms have invariably led to the observation of a tyrosyl radical of... 

Compound A is converted to the P intermediate concomitant with the oxidation of cytochromes a and 03 (Figure 7) at a rate of 3.5 x 10 s [49a, 52]. Even if P is shown as a peroxide (hence its name), the actual structure of this species is still a matter of debate. It has been suggested to be ferryl (Fe +=0 ) cytochrome 03 [53], but the fact that P is formed also in the reaction of the two-electron reduced enzyme with O2 [52c, 54] speaks against this. As alternatives Fe + [55], Cub " " [46a] or even a protein radical [46c,d, 56] have been proposed. It has been reported [46a,d] that no correlation exists between the radical EPR signal and the concentration of any intermediate, including P, but it should be noted that a radical was first observed in... 

Transient absorption measurements show that the spring action of the bisporphyrin cleft does little to impede reclamping to form the p-oxo species, but rather is important to opening the cofacial cleft to allow substrate access to the photogenerated ferryl oxidant (Fig. 26). This photochemical and reactivity data support the contention that the DPD framework is structurally spring loaded... 

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