A complete biomimetic iron-sulfur cubane redox series.

Synthetic iron-sulfur cubanes are models for biological cofactors, which are essential to delineate oxidation states in the more complex enzymatic systems. However, a complete series of [Fe4S4]n complexes spanning all redox states accessible by 1-electron transformations of the individual iron atoms (n = 0-4+) has never been prepared, deterring the methodical comparison of structure and spectroscopic signature. Here, we demonstrate that the use of a bulky arylthiolate ligand promoting the encapsulation of alkali-metal cations in the vicinity of the cubane enables the synthesis of such a series. Characterization by EPR, 57Fe Mössbauer spectroscopy, UV-visible electronic absorption, variable-temperature X-ray diffraction analysis, and cyclic voltammetry reveals key trends for the geometry of the Fe4S4 core as well as for the Mössbauer isomer shift, which both correlate systematically with oxidation state. Furthermore, we confirm the S = 4 electronic ground state of the most reduced member of the series, [Fe4S4]0, and provide electrochemical evidence that it is accessible within 0.82 V from the [Fe4S4]2+ state, highlighting its relevance as a mimic of the nitrogenase iron protein cluster.

Multimodal Spectroscopic Analysis of the Fe-S Clusters of the as-Isolated Escherichia coli SufBC2D Complex.

Iron-sulfur (Fe-S) clusters are essential inorganic cofactors dedicated to a wide range of biological functions, including electron transfer and catalysis. Specialized multiprotein machineries present in all types of organisms support their biosynthesis. These machineries encompass a scaffold protein, on which Fe-S clusters are assembled before being transferred to cellular targets. Here, we describe the first characterization of the native Fe-S cluster of the anaerobically purified SufBC2D scaffold from Escherichia coli by XAS and Mössbauer, UV-visible absorption, and EPR spectroscopies. Interestingly, we propose that SufBC2D harbors two iron-sulfur-containing species, a [2Fe-2S] cluster and an as-yet unidentified species. Mutagenesis and biochemistry were used to propose amino acid ligands for the [2Fe-2S] cluster, supporting the hypothesis that both SufB and SufD are involved in the Fe-S cluster ligation. The [2Fe-2S] cluster can be transferred to ferredoxin in agreement with the SufBC2D scaffold function. These results are discussed in the context of Fe-S cluster biogenesis.

Evidence for [2Fe-2S]2+ and Linear [3Fe-4S]1+ Clusters in a Unique Family of Glycine/Cysteine-Rich Fe-S Proteins from Megavirinae Giant Viruses.

We have discovered a protein with an amino acid composition exceptionally rich in glycine and cysteine residues in the giant virus mimivirus. This small 6 kDa protein is among the most abundant proteins in the icosahedral 0.75 µm viral particles; it has no predicted function but is probably essential for infection. The aerobically purified red-brownish protein overproduced inEscherichia coli contained both iron and inorganic sulfide. UV/vis, EPR, and Mössbauer studies revealed that the viral protein, coined GciS, accommodated two distinct Fe-S clusters: a diamagnetic S = 0 [2Fe-2S]2+ cluster and a paramagnetic S = 5/2 linear [3Fe-4S]1+ cluster, a geometry rarely stabilized in native proteins. Orthologs of mimivirus GciS were identified within all clades of Megavirinae, a Mimiviridae subfamily infecting Acanthamoeba, including the distantly related tupanviruses, and displayed the same spectroscopic features. Thus, these glycine/cysteine-rich proteins form a new family of viral Fe-S proteins sharing unique Fe-S cluster binding properties.

Mixed Valence (µ-Phenoxido) FeII FeIII and FeIII FeIV Compounds: Electron and Proton Transfers.

Mixed-valence non-heme diiron centers are present at the active sites of a few enzymes and confer them interesting reactivities with the two ions acting in concert. Related (µ-phenoxido)diiron complexes have been developed as enzyme mimics. They exhibit very rich spectroscopic properties enabling independent monitoring of each individual ion, which proved useful for mechanistic studies of catalytic hydrolysis and oxidation reactions. In our studies of such complexes, we observed that these compounds give rise to a wide variety of electron transfers (intervalence charge transfer), proton transfers (tautomerism), coupled electron and proton transfers (H. abstraction and PCET). In this minireview, we present and analyze the main results illustrating the latter aspects.

The Ruthenium Nitrosyl Moiety in Clusters: Trinuclear Linear µ-Hydroxido Magnesium(II)-Diruthenium(II), µ3-Oxido Trinuclear Diiron(III)-Ruthenium(II), and Tetranuclear µ4-Oxido Trigallium(III)-Ruthenium(II) Complexes.

The ruthenium nitrosyl moiety, {RuNO}6, is important as a potential releasing agent of nitric oxide and is of inherent interest in coordination chemistry. Typically, {RuNO}6 is found in mononuclear complexes. Herein we describe the synthesis and characterization of several multimetal cluster complexes that contain this unit. Specifically, the heterotrinuclear µ3-oxido clusters [Fe2RuCl4(µ3-O)(µ-OMe)(µ-pz)2(NO)(Hpz)2] (6) and [Fe2RuCl3(µ3-O)(µ-OMe)(µ-pz)3(MeOH)(NO)(Hpz)][Fe2RuCl3(µ3-O)(µ-OMe)(µ-pz)3(DMF)(NO)(Hpz)] (7·MeOH·2H2O) and the heterotetranuclear µ4-oxido complex [Ga3RuCl3(µ4-O)(µ-OMe)3(µ-pz)4(NO)] (8) were prepared from trans-[Ru(OH)(NO)(Hpz)4]Cl2 (5), which itself was prepared via acidic hydrolysis of the linear heterotrinuclear complex {[Ru(µ-OH)(µ-pz)2(pz)(NO)(Hpz)]2Mg} (4). Complex 4 was synthesized from the mononuclear Ru complexes (H2pz)[trans-RuCl4(Hpz)2] (1), trans-[RuCl2(Hpz)4]Cl (2), and trans-[RuCl2(Hpz)4] (3). The new compounds 4-8 were all characterized by elemental analysis, ESI mass spectrometry, IR, UV-vis, and 1H NMR spectroscopy, and single-crystal X-ray diffraction, with complexes 6 and 7 being characterized also by temperature-dependent magnetic susceptibility measurements and Mössbauer spectroscopy. Magnetometry indicated a strong antiferromagnetic interaction between paramagnetic centers in 6 and 7. The ability of 4 and 6-8 to form linkage isomers and release NO upon irradiation in the solid state was investigated by IR spectroscopy. A theoretical investigation of the electronic structure of 6 by DFT and ab initio CASSCF/NEVPT2 calculations indicated a redox-noninnocent behavior of the NO ancillary ligand in 6, which was also manifested in TD-DFT calculations of its electronic absorption spectrum. The electronic structure of 6 was also studied by an X-ray charge density analysis.

Mössbauer Spectroscopic and Computational Investigation of An Iron Cyclopentadienone Complex.

(Cyclopentadienone)iron carbonyl complexes have recently received particular attention for their use as catalysts in hydrogenation or transfer hydrogenation reactions including the N-alkylation of amines with alcohols. This is due to their easy synthesis from simple and cheap materials, air and water stabilities, and the crucial metal-ligand cooperation giving rise to unique catalytic properties. Here, we report a Mössbauer spectroscopic and computational investigation of such a complex and its corresponding activated species for dehydrogenation and hydrogenation reactions. This study affords a deeper understanding of the species formed by the reaction with Me3NO and their distribution upon the added amount of an oxidant.

In†Cellulo Mössbauer and EPR Studies Bring New Evidence to the Long-Standing Debate on Iron-Sulfur Cluster Binding in Human Anamorsin.

Human anamorsin is an iron-sulfur (Fe-S)-cluster-binding protein acting as an electron donor in the early steps of cytosolic iron-sulfur protein biogenesis. Human anamorsin belongs to the eukaryotic CIAPIN1 protein family and contains two highly conserved cysteine-rich motifs, each binding an Fe-S cluster. In vitro works by various groups have provided rather controversial results for the type of Fe-S clusters bound to the CIAPIN1 proteins. In order to unravel the knot on this topic, we used an in cellulo approach combining Mössbauer and EPR spectroscopies to characterize the iron-sulfur-cluster-bound form of human anamorsin. We found that the protein binds two [2Fe-2S] clusters at both its cysteine-rich motifs.

Iron Oxidation in Escherichia coli Bacterioferritin Ferroxidase Centre, a Site Designed to React Rapidly with H2 O2 but Slowly with O2.

Both O2 and H2 O2 can oxidize iron at the ferroxidase center (FC) of Escherichia coli bacterioferritin (EcBfr) but mechanistic details of the two reactions need clarification. UV/Vis, EPR, and Mössbauer spectroscopies have been used to follow the reactions when apo-EcBfr, pre-loaded anaerobically with Fe2+ , was exposed to O2 or H2 O2 . We show that O2 binds di-Fe2+ FC reversibly, two Fe2+ ions are oxidized in concert and a H2 O2 molecule is formed and released to the solution. This peroxide molecule further oxidizes another di-Fe2+ FC, at a rate circa 1000 faster than O2 , ensuring an overall 1:4 stoichiometry of iron oxidation by O2 . Initially formed Fe3+ can further react with H2 O2 (producing protein bound radicals) but relaxes within seconds to an H2 O2 -unreactive di-Fe3+ form. The data obtained suggest that the primary role of EcBfr in vivo may be to detoxify H2 O2 rather than sequester iron.

Evolution of Ate-Organoiron(II) Species towards Lower Oxidation States: Role of the Steric and Electronic Factors.

Ate-iron(II) species such as [Ar3 FeII ]- (Ar=aryl) are key intermediates in Fe-catalyzed couplings between aryl nucleophiles and organic electrophiles. They can be active species in the catalytic cycle, or lead to Fe0 and FeI oxidation states, which can themselves be catalytically active or lead to unwished organic byproducts. Analysis of the reactivity of the intermediates obtained by step-by-step displacement of the mesityl groups in high-spin [Mes3 FeII ]- by less hindered phenyl ligands was performed, and uncovered the crucial role of both steric and electronic parameters in the formation of the Fe0 and FeI oxidation states. The formation of quaternized [Ar4 FeII MgBr(THF)]- intermediates allows the bielectronic reductive elimination energy required for the formation of Fe0 to be reduced. Similarly, the small steric pressure of the aryl groups in [Ar3 FeII ]- enables the formation of aryl-bridged [{FeII (Ar)2 }2 (µ-Ar)2 ]2- species, which afford the FeI oxidation state by bimetallic reductive elimination. These results are supported by 1 H NMR, EPR, and 57 Fe Mössbauer spectroscopies, as well as by DFT calculations.

Single-Ion Magnetic Behaviour in an Iron(III) Porphyrin Complex: A Dichotomy Between High Spin and 5/2-3/2 Spin Admixture.

A mononuclear iron(III) porphyrin compound exhibiting unexpectedly slow magnetic relaxation, which is a characteristic of single-ion magnet behaviour, is reported. This behaviour originates from the close proximity (≈550 cm-1 ) of the intermediate-spin S=3/2 excited states to the high-spin S=5/2 ground state. More quantitatively, although the ground state is mostly S=5/2, a spin-admixture model evidences a sizable contribution (≈15 %) of S=3/2 to the ground state, which as a consequence experiences large and positive axial anisotropy (D=+19.2 cm-1 ). Frequency-domain EPR spectroscopy allowed the mS = |±1/2⟩→|±3/2⟩ transitions to be directly accessed, and thus the very large zero-field splitting in this 3d5 system to be unambiguously measured. Other experimental results including magnetisation, Mössbauer, and field-domain EPR studies are consistent with this model, which is also supported by theoretical calculations.

Development of a Rubredoxin-Type Center Embedded in a de Dovo-Designed Three-Helix Bundle.

Protein design is a powerful tool for interrogating the basic requirements for the function of a metal site in a way that allows for the selective incorporation of elements that are important for function. Rubredoxins are small electron transfer proteins with a reduction potential centered near 0 mV (vs normal hydrogen electrode). All previous attempts to design a rubredoxin site have focused on incorporating the canonical CXXC motifs in addition to reproducing the peptide fold or using flexible loop regions to define the morphology of the site. We have produced a rubredoxin site in an utterly different fold, a three-helix bundle. The spectra of this construct mimic the ultraviolet-visible, Mössbauer, electron paramagnetic resonance, and magnetic circular dichroism spectra of native rubredoxin. Furthermore, the measured reduction potential suggests that this rubredoxin analogue could function similarly. Thus, we have shown that an α-helical scaffold sustains a rubredoxin site that can cycle with the desired potential between the Fe(II) and Fe(III) states and reproduces the spectroscopic characteristics of this electron transport protein without requiring the classic rubredoxin protein fold.

Contribution of Mössbauer spectroscopy to the investigation of Fe/S biogenesis.

Fe/S cluster biogenesis involves a complex machinery comprising several mitochondrial and cytosolic proteins. Fe/S cluster biosynthesis is closely intertwined with iron trafficking in the cell. Defects in Fe/S cluster elaboration result in severe diseases such as Friedreich ataxia. Deciphering this machinery is a challenge for the scientific community. Because iron is a key player, 57Fe-Mössbauer spectroscopy is especially appropriate for the characterization of Fe species and monitoring the iron distribution. This minireview intends to illustrate how Mössbauer spectroscopy contributes to unravel steps in Fe/S cluster biogenesis. Studies were performed on isolated proteins that may be present in multiple protein complexes. Since a few decades, Mössbauer spectroscopy was also performed on whole cells or on isolated compartments such as mitochondria and vacuoles, affording an overview of the iron trafficking. This minireview aims at presenting selected applications of 57Fe-Mössbauer spectroscopy to Fe/S cluster biogenesis.

Correction to: Contribution of Mössbauer spectroscopy to the investigation of Fe/S biogenesis.

The article "Contribution of Mössbauer spectroscopy to the investigation of Fe/S biogenesis", written by Ricardo Garcia­Serres, Martin Clémancey, Jean­Marc Latour, Geneviève Blondin was originally published electronically on the publisher's internet portal (currently SpringerLink) without open access.

Redox Control of the Human Iron-Sulfur Repair Protein MitoNEET Activity via Its Iron-Sulfur Cluster.

Human mitoNEET (mNT) is the first identified Fe-S protein of the mammalian outer mitochondrial membrane. Recently, mNT has been implicated in cytosolic Fe-S repair of a key regulator of cellular iron homeostasis. Here, we aimed to decipher the mechanism by which mNT triggers its Fe-S repair capacity. By using tightly controlled reactions combined with complementary spectroscopic approaches, we have determined the differential roles played by both the redox state of the mNT cluster and dioxygen in cluster transfer and protein stability. We unambiguously demonstrated that only the oxidized state of the mNT cluster triggers cluster transfer to a generic acceptor protein and that dioxygen is neither required for the cluster transfer reaction nor does it affect the transfer rate. In the absence of apo-acceptors, a large fraction of the oxidized holo-mNT form is converted back to reduced holo-mNT under low oxygen tension. Reduced holo-mNT, which holds a [2Fe-2S](+)with a global protein fold similar to that of the oxidized form is, by contrast, resistant in losing its cluster or in transferring it. Our findings thus demonstrate that mNT uses an iron-based redox switch mechanism to regulate the transfer of its cluster. The oxidized state is the "active state," which reacts promptly to initiate Fe-S transfer independently of dioxygen, whereas the reduced state is a "dormant form." Finally, we propose that the redox-sensing function of mNT is a key component of the cellular adaptive response to help stress-sensitive Fe-S proteins recover from oxidative injury.

Structural Insights into the Nature of Fe0 and FeI Low-Valent Species Obtained upon the Reduction of Iron Salts by Aryl Grignard Reagents.

Mechanistic studies of the reduction of FeIII and FeII salts by aryl Grignard reagents in toluene/tetrahydrofuran mixtures in the absence of a supporting ligand, as well as structural insights regarding the nature of the low-valent iron species obtained at the end of this reduction process, are reported. It is shown that several reduction pathways can be followed, depending on the starting iron precursor. We demonstrate, moreover, that these pathways lead to a mixture of Fe0 and FeI complexes regardless of the nature of the precursor. Mössbauer and 1H NMR spectroscopies suggest that diamagnetic 16-electron bisarene complexes such as (η4-C6H5Me)2Fe0 can be formed as major species (85% of the overall iron quantity). The formation of a η6-arene-ligated low-spin FeI complex as a minor species (accounting for ca. 15% of the overall iron quantity) is attested by Mössbauer spectroscopy, as well as by continuous-wave electron paramagnetic resonance (EPR) and pulsed-EPR (HYSCORE) spectroscopies. The nature of the FeI coordination sphere is discussed by means of isotopic labeling experiments and density functional theory calculations. It is shown that the most likely low-spin FeI candidate obtained in these systems is a diphenylarene-stabilized species [(η6-C6H5Me)FeIPh2]- exhibiting an idealized C2v topology. This enlightens the nature of the lowest valence states accommodated by iron during the reduction of FeIII and FeII salts by aryl Grignard reagents in the absence of any additional coligand, which so far remained rather unknown. The reactivity of these low-valent FeI and Fe0 complexes in aryl-heteroaryl Kumada cross-coupling conditions has also been investigated, and it is shown that the zerovalent Fe0 species can be used efficiently as a precursor in this reaction, whereas the FeI oxidation state does not exhibit any reactivity.

Intramolecular Electron Transfers Thwart Bistability in a Pentanuclear Iron Complex.

With the intention to investigate the redox properties of polynuclear complexes as previously reported for the pentamanganese complex [{Mn(II)(µ-bpp)3}2Mn(III)Mn(II)2(µ3-O)](3+) (2(3+)), we focused on the analogous pentairon complex that was previously isolated as all-ferrous. As Masaoka and co-workers recently published, aerobic synthesis leads to the [{Fe(II)(µ-bpp)3}2Fe(III)Fe(II)2(µ3-O)](3+) complex (1(3+)). This species exhibits in acetonitrile solution four reversible one-electron oxidation waves. Accordingly, the three oxidized species 1(4+), 1(5+), and 1(6+) with a 3Fe(II)2Fe(III), 2Fe(II)3Fe(III), and 1Fe(II)4Fe(III) composition, respectively, were generated by bulk electrolysis and isolated. Mössbauer spectroscopy allowed us to determine the spin states of all the iron ions and to unambiguously locate the sites of the successive oxidations. They all occur in the µ3-oxo core except for the 1(4+) to 1(5+) process that presents a striking electronic rearrangement, with both metals in axial position being oxidized while the core is reduced to the [Fe(III)Fe(II)2(µ3-O)](5+) oxidation level. This strongly differs from the redox behavior of the Mn5 system. The origin of this electronic switch is discussed.

Phosphoester hydrolysis: the incoming substrate turns the bridging hydroxido nucleophile into a terminal one.

Identifying the active nucleophile in hydrolysis reactions catalyzed by binuclear hydrolases is a recurrent problem and a matter of intense debate. We report on the phosphate ester hydrolysis by a Fe(III)Fe(II) complex of a binucleating ligand. This complex presents activities in the range of those observed for similar biomimetic compounds in the literature. The specific electronic properties of the Fe(III)Fe(II) complex allowed us to use (1)H NMR and Mössbauer spectroscopies to investigate the nature of the various species present in the solution in the pH range of 5-10. Both techniques showed that the hydrolysis activity is associated to a µ-hydroxido Fe(III)Fe(II) species. Further (1)H NMR experiments show that binding of anions or the substrate changes this bonding mode suggesting that a terminal hydroxide is the likely nucleophile in these hydrolysis reactions. This view is further supported by the structure determination of the hydrolysis product.

Deprotonation in Mixed-Valent Diiron(II,III) Complexes with Aniline or Benzimidazole Ligands.

We have previously investigated cis/trans isomerization processes in phenoxido-bridged mixed-valent Fe(II)Fe(III) complexes that contain either one aniline or one anilide ligand. In this work, we compare the properties of similar complexes bearing one terminal protic ligand, either aniline or 1H-benzimidazole. Whatever the ligand, (1)H NMR spectroscopy clearly evidences that the complexes are present in CH3CN as a mixture of cis- and trans-isomers in a close to 1:1 ratio. We show here that addition of NEt3 indeed allows the deprotonation of these ligands, the resulting complexes bearing either anilide or benzimidazolide that are coordinated to the ferric site. The latter are singular examples of a high-spin ferric ion coordinated to a benzimidazolide ligand. Whereas the trans-isomer of the anilide complex is the overwhelming species, benzimidazolide species are mixtures of cis- and trans-isomers in equal proportions. Moreover, cyclic voltammametry studies show that Fe(III)Fe(III) complexes with 1H-benzimidazole are more stable than their aniline counterparts, whereas the reverse is observed for the deprotonated species.

Dinitrogen activation upon reduction of a triiron(II) complex.

Reaction of a trinuclear iron(II) complex, Fe3 Br3 L (1), with KC8 under N2 leads to dinitrogen activation products (2) from which Fe3 (NH)3 L (2-1; L is a cyclophane bridged by three ß-diketiminate arms) was characterized by X-ray crystallography. (1) H NMR spectra of the protonolysis product of 2 synthesized under (14) N2 and (15) N2 confirm atmospheric N2 reduction, and ammonia is detected by the indophenol assay (yield ∼30 %). IR and Mössbauer spectroscopy, and elemental analysis on 2 and 2-1 as well as the tri(amido)triiron(II) 3 and tri(methoxo)triiron 4 congeners support our assignment of the reduction product as containing protonated N-atom bridges.

Triggering the generation of an iron(IV)-oxo compound and its reactivity toward sulfides by Ru(II) photocatalysis.

The preparation of [Fe(IV)(O)(MePy2tacn)](2+) (2, MePy2tacn = N-methyl-N,N-bis(2-picolyl)-1,4,7-triazacyclononane) by reaction of [Fe(II)(MePy2tacn)(solvent)](2+) (1) and PhIO in CH3CN and its full characterization are described. This compound can also be prepared photochemically from its iron(II) precursor by irradiation at 447 nm in the presence of catalytic amounts of [Ru(II)(bpy)3](2+) as photosensitizer and a sacrificial electron acceptor (Na2S2O8). Remarkably, the rate of the reaction of the photochemically prepared compound 2 toward sulfides increases 150-fold under irradiation, and 2 is partially regenerated after the sulfide has been consumed; hence, the process can be repeated several times. The origin of this rate enhancement has been established by studying the reaction of chemically generated compound 2 with sulfides under different conditions, which demonstrated that both light and [Ru(II)(bpy)3](2+) are necessary for the observed increase in the reaction rate. A combination of nanosecond time-resolved absorption spectroscopy with laser pulse excitation and other mechanistic studies has led to the conclusion that an electron transfer mechanism is the most plausible explanation for the observed rate enhancement. According to this mechanism, the in-situ-generated [Ru(III)(bpy)3](3+) oxidizes the sulfide to form the corresponding radical cation, which is eventually oxidized by 2 to the corresponding sulfoxide.