FeIII OOH Research Articles

Stability and Catalase-Like Activity of a Mononuclear Non-Heme Oxoiron(IV) Complex in Aqueous Solution.

Heme-type catalase is a class of oxidoreductase enzymes responsible for the biological defense against oxidative damage of cellular components caused by hydrogen peroxide, where metal-oxo species are proposed as reactive intermediates. To get more insight into the mechanism of this curious reaction a non-heme structural and functional model was carried out by the use of a mononuclear complex [FeII(N4Py*)(CH3CN)](CF3SO3)2 (N4Py* = N,N-bis(2-pyridylmethyl)- 1,2-di(2-pyridyl)ethylamine) as a catalyst, where the possible reactive intermediates, high-valent FeIV=O and FeIII–OOH are known and spectroscopically well characterized. The kinetics of the dismutation of H2O2 into O2 and H2O was investigated in buffered water, where the reactivity of the catalyst was markedly influenced by the pH, and it revealed Michaelis–Menten behavior with KM = 1.39 M, kcat = 33 s−1 and k2(kcat/KM) = 23.9 M−1s−1 at pH 9.5. A mononuclear [(N4Py)FeIV=O]2+ as a possible intermediate was also prepared, and the pH dependence of its stability and reactivity in aqueous solution against H2O2 was also investigated. Based on detailed kinetic, and mechanistic studies (pH dependence, solvent isotope effect (SIE) of 6.2 and the saturation kinetics for the initial rates versus the H2O2 concentration with KM = 18 mM) lead to the conclusion that the rate-determining step in these reactions above involves hydrogen-atom transfer between the iron-bound substrate and the Fe(IV)-oxo species.

Oxidation chemistry of C–H bond by mononuclear iron complexes derived from tridentate ligands containing phenolato function

A family of phenolato-based nonheme mononuclear iron(III) complexes, [FeIII(Ln)Cl2](1–4) (n=1, 2, 3, and 4) derived from tridentate ligands LnH (where L1H=2-((2-phenyl-2-(pyridine-2-yl)hydrazono)methylphenol, L2H=2-((2-phenyl-2-(pyridine-2-yl)hyrazono)nitrophenol, L3H=4-methoxy-2-((2-phenyl-2-(pyridin-2-yl)hydrazono) methyl) phenol and L4H=2,4-di-tert-butyl-6-((2-phenyl-2-(pyridin-2-yl)hydrazono)methyl)phenol and H was dissociable proton) were synthesized and characterized by different spectroscopic methods. The molecular structure of [Fe(L4)Cl2](4) was determined by X-ray crystallography. The electronic structures of complexes 2 and 4 were investigated by DFT and TD-DFT calculations. All complexes were employed as catalyst for C–H bond oxidation utilizing H2O2 as an oxidant. Effect of the substituents on the phenyl ring containing phenolato function was scrutinized for efficiency and selectivity of the oxidation process. Role of key intermediate FeIII–OOH and FeIVO was also be described.

Demonstration of the Heterolytic OO Bond Cleavage of Putative Nonheme Iron(II)OOH(R) Complexes for Fenton and Enzymatic Reactions

AbstractOne‐electron reduction of mononuclear nonheme iron(III) hydroperoxo (FeIIIOOH) and iron(III) alkylperoxo (FeIIIOOR) complexes by ferrocene (Fc) derivatives resulted in the formation of the corresponding iron(IV) oxo complexes. The conversion rates were dependent on the concentration and oxidation potentials of the electron donors, thus indicating that the reduction of the iron(III) (hydro/alkyl)peroxo complexes to their one‐electron reduced iron(II) (hydro/alkyl)peroxo species is the rate‐determining step, followed by the heterolytic OO bond cleavage of the putative iron(II) (hydro/alkyl)peroxo species to give the iron(IV) oxo complexes. Product analysis supported the heterolytic OO bond‐cleavage mechanism. The present results provide the first example showing the one‐electron reduction of iron(III) (hydro/alkyl)peroxo complexes and the heterolytic OO bond cleavage of iron(II) (hydro/alkyl)peroxo species to form iron(IV) oxo intermediates which occur in nonheme iron enzymatic and Fenton reactions.

Demonstration of the heterolytic O-O bond cleavage of putative nonheme iron(II)-OOH(R) complexes for Fenton and enzymatic reactions.

One-electron reduction of mononuclear nonheme iron(III) hydroperoxo (Fe(III)-OOH) and iron(III) alkylperoxo (Fe(III)-OOR) complexes by ferrocene (Fc) derivatives resulted in the formation of the corresponding iron(IV) oxo complexes. The conversion rates were dependent on the concentration and oxidation potentials of the electron donors, thus indicating that the reduction of the iron(III) (hydro/alkyl)peroxo complexes to their one-electron reduced iron(II) (hydro/alkyl)peroxo species is the rate-determining step, followed by the heterolytic O-O bond cleavage of the putative iron(II) (hydro/alkyl)peroxo species to give the iron(IV) oxo complexes. Product analysis supported the heterolytic O-O bond-cleavage mechanism. The present results provide the first example showing the one-electron reduction of iron(III) (hydro/alkyl)peroxo complexes and the heterolytic O-O bond cleavage of iron(II) (hydro/alkyl)peroxo species to form iron(IV) oxo intermediates which occur in nonheme iron enzymatic and Fenton reactions.

Heme utilization by pathogenic bacteria: not all pathways lead to biliverdin.

ConspectusThe eukaryotic heme oxygenases (HOs) (E.C. 1.14.99.3) convert heme to biliverdin, iron, and carbon monoxide (CO) in three successive oxygenation steps. Pathogenic bacteria require iron for survival and infection. Extracellular heme uptake from the host plays a critical role in iron acquisition and virulence. In the past decade, several HOs required for the release of iron from extracellular heme have been identified in pathogenic bacteria, including Corynebacterium diphtheriae, Neisseriae meningitides, and Pseudomonas aeruginosa. The bacterial enzymes were shown to be structurally and mechanistically similar to those of the canonical eukaryotic HO enzymes. However, the recent discovery of the structurally and mechanistically distinct noncanonical heme oxygenases of Staphylococcus aureus and Mycobacterium tuberculosis has expanded the reaction manifold of heme degradation. The distinct ferredoxin-like structural fold and extreme heme ruffling are proposed to give rise to the alternate heme degradation products in the S. aureus and M. tuberculosis enzymes. In addition, several “heme-degrading factors” with no structural homology to either class of HOs have recently been reported. The identification of these “heme-degrading proteins” has largely been determined on the basis of in vitro heme degradation assays. Many of these proteins were reported to produce biliverdin, although no extensive characterization of the products was performed. Prior to the characterization of the canonical HO enzymes, the nonenzymatic degradation of heme and heme proteins in the presence of a reductant such as ascorbate or hydrazine, a reaction termed “coupled oxidation”, served as a model for biological heme degradation. However, it was recognized that there were important mechanistic differences between the so-called coupled oxidation of heme proteins and enzymatic heme oxygenation. In the coupled oxidation reaction, the final product, verdoheme, can readily be converted to biliverdin under hydrolytic conditions. The differences between heme oxygenation by the canonical and noncanonical HOs and coupled oxidation will be discussed in the context of the stabilization of the reactive FeIII–OOH intermediate and regioselective heme hydroxylation. Thus, in the determination of heme oxygenase activity in vitro, it is important to ensure that the reaction proceeds through successive oxygenation steps. We further suggest that when bacterial heme degradation is being characterized, a systems biology approach combining genetics, mechanistic enzymology, and metabolite profiling should be undertaken.

A DFT Study of the cis-Dihydroxylation of Nitroaromatic Compounds Catalyzed by Nitrobenzene Dioxygenase

The mechanism of cis-dihydroxylation of nitrobenzene and 2-nitrotoluene catalyzed by nitrobenzene 1,2-dioxygenase (NBDO), a member of the naphthalene family of Rieske non-heme iron dioxygenases, was studied by means of the density functional theory method using four models of the enzyme active site. Different possible reaction pathways for the substrate dioxygenation initiated either by the FeIII–OOH or HO–FeV=O attack on the aromatic ring were considered and the computed activation barriers compared with the Gibbs free energy of activation for the oxygen–oxygen cleavage leading to the formation of the iron(V)–oxo species from its ferric hydroperoxo precursor. The mechanism of the substrate cis-dihydroxylation leading to the formation of a cis-dihydrodiol was then investigated, and the most feasible mechanism was found to be starting with the attack of the high-valent iron–oxo species on the substrate ring yielding a radical intermediate, which further evolves toward the final product.

Dataset for Modelling Reaction Mechanisms Using Density Functional Theory: Mechanism of ortho‐Hydroxylation by High‐Valent Iron‐Oxo Species

Modelling reaction mechanisms using density functional theory is one of the popular routes to underpin the course of a chemical reaction. Although numerous publications have come out in this area, the pitfall of modelling such reactions and explicitly publishing the entire data set (structures, energies, coordinates, spin densities, etc.) which lead to the conclusions are scarce. Here we have attempted to set a trend wherein all the computed data to underpin the reaction mechanism of ortho‐hydroxylation of aromatic compounds by high‐valent iron‐oxo complexes (FeIII–OOH, FeIV=O, and FeV=O) are collected. Since the structure, energetics and other details of the calculations can be employed in future to probe/understand the reactivity pattern of such species, establishing the data set is justified. Here by analysing the presented results we also discuss in brief the presented results.

Geometric and Electronic Structure Contributions to Function in Non-heme Iron Enzymes

Mononuclear non-heme Fe (NHFe) enzymes play key roles in DNA repair, the biosynthesis of antibiotics, the response to hypoxia, cancer therapy, and many other biological processes. These enzymes catalyze a diverse range of oxidation reactions, including hydroxylation, halogenation, ring closure, desaturation, and electrophilic aromatic substitution (EAS). Most of these enzymes use an Fe(II) site to activate dioxygen, but traditional spectroscopic methods have not allowed researchers to insightfully probe these ferrous active sites. We have developed a methodology that provides detailed geometric and electronic structure insights into these NHFe(II) active sites. Using these data, we have defined a general mechanistic strategy that many of these enzymes use: they control O2 activation (and limit autoxidation and self-hydroxylation) by allowing Fe(II) coordination unsaturation only in the presence of cosubstrates. Depending on the type of enzyme, O2 activation either involves a 2e(-) reduced Fe(III)-OOH intermediate or a 4e(-) reduced Fe(IV)═O intermediate. Nuclear resonance vibrational spectroscopy (NRVS) has provided the geometric structure of these intermediates, and magnetic circular dichroism (MCD) has defined the frontier molecular orbitals (FMOs), the electronic structure that controls reactivity. This Account emphasizes that experimental spectroscopy is critical in evaluating the results of electronic structure calculations. Therefore these data are a key mechanistic bridge between structure and reactivity. For the Fe(III)-OOH intermediates, the anticancer drug activated bleomycin (BLM) acts as the non-heme Fe analog of compound 0 in heme (e.g., P450) chemistry. However BLM shows different reactivity: the low-spin (LS) Fe(III)-OOH can directly abstract a H atom from DNA. The LS and high-spin (HS) Fe(III)-OOHs have fundamentally different transition states. The LS transition state goes through a hydroxyl radical, but the HS transition state is activated for EAS without O-O cleavage. This activation is important in one class of NHFe enzymes that utilizes a HS Fe(III)-OOH intermediate in dioxygenation. For Fe(IV)═O intermediates, the LS form has a π-type FMO activated for attack perpendicular to the Fe-O bond. However, the HS form (present in the NHFe enzymes) has a π FMO activated perpendicular to the Fe-O bond and a σ FMO positioned along the Fe-O bond. For the NHFe enzymes, the presence of π and σ FMOs enables enzymatic control in determining the type of reactivity: EAS or H-atom extraction for one substrate with different enzymes and halogenation or hydroxylation for one enzyme with different substrates.

ChemInform Abstract: Active Species of Nonheme Iron and Manganese‐Catalyzed Oxidations

AbstractReview: 93 refs.

A Mononuclear Non-Heme High-Spin Iron(III)–Hydroperoxo Complex as an Active Oxidant in Sulfoxidation Reactions

We report the first direct experimental evidence showing that a high-spin iron(III)-hydroperoxo complex bearing an N-methylated cyclam ligand can oxidize thioanisoles. DFT calculations showed that the reaction pathway involves heterolytic O-O bond cleavage and that the choice of the heterolytic pathway versus the homolytic pathway is dependent on the spin state and the number of electrons in the d(xz) orbital of the Fe(III)-OOH species.

Active Species of Nonheme Iron and Manganese-Catalyzed Oxidations

The intense catalytic and spectroscopic studies of the last decade provided important insights into the mechanisms of some nonheme iron-catalyzed oxidations with hydrogen peroxide. For manganese-based analogs, direct spectroscopic data on the structure of the reactive intermediates are scarce; mechanistic proposals are mainly based on catalytic studies and on analogy with iron systems. Herein, these data are summarized and contemporary mechanistic landscape is presented. We have mainly focused on iron and manganese complexes with N4-donor aminopyridine ligands, which are one of the most successful catalysts for chemo-, regio- and enantioselective transformations of organic substrates with H2O2 and H2O2/CH3COOH as oxidants. The low-spin FeIII–OOH, FeIV = O and FeV = O species can be spectroscopically trapped in the catalyst systems studied, low-spin FeV = O intermediate being the most likely key oxidizing agent.

Unraveling the Mechanism of Catalytic Reduction of O2 by Microperoxidase-11 Adsorbed within a Transparent 3D-Nanoporous ITO Film

Nanoporous films of indium tin oxide (ITO), with thicknesses ranging from 250 nm to 2 μm, were prepared by Glancing Angle Deposition (GLAD) and used as highly sensitive transparent 3D-electrodes for quantitatively interrogating, by time-resolved spectroelectrochemistry, the reactivity of microperoxidase-11 (MP-11) adsorbed within such films. The capacitive current densities of these 3D-electrodes as well as the amount of adsorbed MP-11 were shown to be linearly correlated to the GLAD ITO film thickness, indicating a homogeneous distribution of MP-11 across the film as well as homogeneous film porosity. Under saturating adsorption conditions, MP-11 film concentration as high as 60 mM was reached. This is equivalent to a stack of 110 monolayers of MP-11 per micrometer film thickness. This high MP-11 film loading combined with the excellent ITO film conductivity has allowed the simultaneous characterization of the heterogeneous one-electron transfer dynamics of the MP-11 Fe(III)/Fe(II) redox couple by cyclic voltammetry and cyclic voltabsorptometry, up to a scan rate of few volts per second with a satisfactory single-scan signal-to-noise ratio. The potency of the method to unravel complex redox coupled chemical reactions was also demonstrated with the catalytic reduction of oxygen by MP-11. In the presence of O(2), cross-correlation of electrochemical and spectroscopic data has allowed us to determine the key kinetics and thermodynamics parameters of the redox catalysis that otherwise could not be easily extracted using conventional protein film voltammetry. On the basis of numerical simulations of cyclic voltammograms and voltabsorptograms and within the framework of different plausible catalytic reaction schemes including appropriate approximations, it was shown possible to discriminate between different possible catalytic pathways and to identify the relevant catalytic cycle. In addition, from the best fits of simulations to the experimental voltammograms and voltabsorptograms, the partition coefficient of O(2) for the ITO film as well as the values of two kinetic rate constants could be extracted. It was finally concluded that the catalytic reduction of O(2) by MP-11 adsorbed within nanoporous ITO films occurs via a 2-electron mechanism with the formation of an intermediate Fe(III)-OOH adduct characterized by a decay rate of 11 s(-1). The spectroelectroanalytical strategy presented here opens new opportunities for characterizing complex redox-coupled chemical reactions not only with redox proteins, but also with redox biomimetic systems and catalysts. It might also be of great interest for the development and optimization of new spectroelectrochemical sensors and biosensors, or eventually new photoelectrocatalytic systems or biofuel cells.

Selenite retention by nanocrystalline magnetite: Role of adsorption, reduction and dissolution/co-precipitation processes

We studied selenite ( SeO 3 2 - ) retention by magnetite ( Fe II Fe III 2 O 4 ) using both surface complexation modeling and X-ray absorption spectroscopy (XAS) to characterize the processes of adsorption, reduction, and dissolution/co-precipitation. The experimental sorption results for magnetite were compared to those of goethite (Fe IIIOOH) under similar conditions. Selenite sorption was investigated under both oxic and anoxic conditions and as a function of pH, ionic strength, solid-to-liquid ratio and Se concentration. Sorption onto both oxides was independent of ionic strength and decreased as pH increased, as expected for anion sorption; however, the shape of the sorption edges was different. The goethite sorption data could be modeled assuming the formation of an inner-sphere complex with iron oxide surface sites (SOH). In contrast, the magnetite sorption data at low pH could be modeled only when the dissolution of magnetite, the formation of aqueous iron–selenite species, and the subsequent surface complexation of these species were implemented. The precipitation of ferric selenite was the predominant retention process at higher selenite concentrations (>1 × 10 −4 M) and pH < 5, which was in agreement with the XAS results. Sorption behavior onto magnetite was similar under oxic and anoxic conditions. Under anoxic conditions, we did not observe the reduction of selenite. Possible reasons for the absence of reduction are discussed. In conclusion, we show that under acidic reaction conditions, selenite retention by magnetite is largely influenced by dissolution and co-precipitation processes.

Non-heme iron polyazadentate complexes as catalysts for aromatic hydroxylation by H 2O 2: Particular efficiency of tetrakis(2-pyridylmethyl)ethylenediamine–iron(II) complexes

Two iron(II) complexes with ligands related to TPEN, N, N, N′, N′-tetrakis-(2-pyridylmethyl)ethane-1,2-diamine, were prepared, characterized and compared as catalysts for the hydroxylation of aromatic compounds by H 2O 2. The (TPEN)Fe II complex gave the best yields ever reported for such iron polyazadentate catalysts with yields up to 86, 59, 59 and 38% for the hydroxylation of anisole, benzene, chlorobenzene and ethylbenzene, respectively. Experiments using H 2 18O 2 and H 2 18O clearly showed that the oxygen atom incorporated in the aromatic substrates almost exclusively comes from H 2O 2. Preliminary experiments on the effects of O 2 and spin-traps on the reactions suggested a minor contribution of uncontrolled radicals such as OH. From the presently available data, a tentative mechanism based on the hydroxylation of substrates by a Fe IIIOOH intermediate or, more probably, by a species resulting from the O–O bond cleavage of this intermediate, such as [Fe IVO + OH], is proposed.

Heme oxygenase, steering dioxygen activation toward heme hydroxylation

The activation of dioxygen by heme oxygenase proceeds via formation of an obligatory ferric hydroperoxide intermediate (Fe III–OOH), as is the case in the activation of dioxygen by monooxygenase enzymes. This review summarizes current understanding of the structural and dynamic properties in heme oxygenase that channel the reactivity of the Fe III-OOH intermediate toward heme hydroxylation rather than oxoferryl formation. In addition, structural and electronic factors dictating the regiospecificity of heme oxygenation are analyzed in the context of recent X-ray and NMR spectroscopic studies. Differences in mechanism between heme hydroxylation, as carried out by heme oxygenase, and the coupled oxidation process, are also addressed.

Spin state tuning of non-heme iron-catalyzed hydrocarbon oxidations: participation of FeIII–OOH and FeVO intermediatesBased on the presentation given at Dalton Discussion No. 4, 10–13th January 2002, Kloster Banz, Germany.

We have found a family of non-heme iron complexes [FeII(L)(CH3CN)2] (L = tetradentate pyridine containing ligand) with cis labile sites that catalyze highly stereoselective hydrocarbon oxidations using H2O2 as oxidant. The hydrocarbon oxidation reactivity patterns of this family of catalysts divide them into two subgroups: Category A catalysts which carry out highly stereoselective alkane hydroxylation, olefin epoxidation, and olefin cis-dihydroxylation via low-spin FeIII–OOH, FeVO intermediates and Category B catalysts which form high-spin FeIII–OOH intermediates and strongly favor olefin cis-dihydroxylation in which both diol oxygen atoms derive from H2O2. 6-Methyl substituents on the ligands play an important role in tuning the spin states of the iron centers to afford a family of non-heme iron complexes that catalyze a remarkable array of hydrocarbon oxidation reactions.

Aromatic hydroxylation by fenton reagents {Reactive intermediate[L x+Fe IIIOOH(BH +)], not free hydroxyl radical (HO·)}

Several iron complexes [Fe II(bpy) 2 2+, Fe II(OPPh 3) 4 2+, and Fe II(PA) 2] in combination with hydrogen peroxide (HOOH) catalytically hydroxylate aromatic substrates (ArH). The base-induced nucleophilic addition of HOOH to the electrophilic iron center yields the reactive intermediate of Fenton reagents ▪. The latter includes a ‘stabilized’ hydroxyl radical that is able to replace an aromatic hydrogen (H) with a hydroxyl group (HO) via an initial addition reaction. With PhCH 3 and PhCH 2CH 3 as substrates free HO· (from the radiolysis of H 2O) reacts via aryl addition (97 and 85%, respectively) to give Ar-Ar as the predominant product, whereas 1 favors H-atom abstraction from the alkyl group (50 and 80%, respectively) and the only detectable products from aryl addition are the respective substituted phenols ( o:p-ArOH). Other substituted benzenes (PhX) undergo addition by free HO at the ortho and para aryl carbons ( o:p ratio, 2), followed by dimerization and elimination of two H 2O molecules to yield substituted biphenyls. In contrast, 1 reacts with PhX to yield substituted phenol (ArOH; o:p ratio, 0.5 – 1.1). With phenol (PhOH) as the substrate, reaction with 1 yields mainly catechol ( o-Ar(OH) 2; o:p ratio, 20). In a solvent matrix of MeCN:H 2O (3:1 mol:mol ratio) the reaction efficiencies with Fe II(bpy) 2 2+ and Fe II(OPPh 3) 4 2+ for the hydroxylation of benzene to phenol are 36 and 42%, respectively (product per HOOH).

The mechanism of cytochrome P-450 dependent C–C bond cleavage: studies on 17α-hydroxylase-17,20-lyase

It is shown that formation of 17α-hydroxyandrost-5-en-3β-ol from pregnenolone occurs solely at the expense of the cleavage of the C-17–C-20 bond of the precursor and is best rationalised by invoking the participation of an FeIII–OOH intermediate in the acyl–carbon fission process.