Diiron-peroxo complexes

Peroxo-diiron(III) complexes can undergo not only redox but also ligand substitution reactions. Liberation of H202 was observed in the reactions with phenols and carboxylic acids leading also to the respective phenolate or carboxylate iron(III) complexes.86 Hydrolysis of a peroxo-diiron(III) complex results in an oxo-diiron(III) species and hydrogen peroxide. Such reaction is responsible for the autoxidation of hemerythrin, but is very slow for the native protein due to hydrophobic shielding of the active site (Section 4.2.3).20 The hydrolysis of iron(III) peroxides is reversible, and the reverse reaction, the formation of peroxo intermediates from H202 and the (di)iron(III), is often referred to as peroxide shunt and is much better studied for model complexes. 

Dioxygen diiron complexes have been synthesized by mimicking the probable intermediate structures in methane monooxygenase oxygenations. Examples are summarized in the recent review. One is the Fc2(i7-0)2 diamond core type and the other is the Fe2(tJ-02) peroxo type. Recently, an unusual Fe"-X-Fe (tl -02) species has been suggested in the reaction of O2 with carboxylate-bridged diiron(II,II) paddlewheel complexes, but its role in the oxygenation has not been clarified. ... 

There have been advances in the structural and physical properties of diiron complexes with dioxygen ligands. All of the well-defined structures have a, u-l,2-peroxo coordinated between two Fe(III) centers. These systems are not directly related to hemerythrin because of the mode of O2 bonding they are more relevant to the proposed structures in the active sites of ribonucleotide reductase (RNR) [5, 15] and MMOH [5] - two nonheme diiron-containing enzymes that activate dioxygen (vide infra). 

Fig. 3. Proposed routes for conversion of the peroxo intermediate to intermediate Q, one involving loss of water (left-hand side) and one not (right-hand side). In the former case the resulting diiron(IV) oxo species could bind an oxygen atom with one iron, or the oxygen could be bound symmetrically by both iron atoms. Although written as an iron(IV) oxo species, Q can also be formulated as an iron(III) oxyl radical complex (35,51).

Fig. 4. Substrate first binds to the complete system containing all three protein components. Addition of NADH next effects two-electron reduction of the hydroxylase from the oxidized Fe(III)Fe(III) to the fully reduced Fe(II)Fe(II) form, bypassing the inactive Fe(II)Fe(III) state. The fully reduced hydroxylase then reacts with dioxygen in a two-electron step to form the first known intermediate, a diiron(III) peroxo complex. The possibility that this species itself is sufficiently activated to carry out the hydroxylation reaction for some substrates cannot be ruled out. The peroxo intermediate is then converted to Q as shown in Fig. 3. Substrate reacts with Q, and product is released with concomitant formation of the diiron(III) form of the hydroxylase, which enters another cycle in the catalysis.

First two complexes with a (p.-oxo)(p-hydroxo)diiron (III) core [Fe2(0)(0H)(6TLA)2(C104)3] (I) and [Fe2(0)2(6TLA)2(C104)2] (II), were isolated and characterized (Zang et al 1995). Structure of a (-l,2-peroxo)bis(-carboxylato)diiron(m)model for the peroxo intermediate in the methane monooxygenase hydroxylase reaction cycle is presented in Fig, 6.3. 

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