Iron oxidation in Escherichia coli bacterioferritin ferroxidase centre, a site designed to react rapidly with H2O2 but slowly with O2

Both O2 and H2O2 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 H2O2. We show that O2 binds di‐Fe2+ FC reversibly, two Fe2+ ions are oxidized in concert and a H2O2 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 H2O2 (producing protein bound radicals) but relaxes within seconds to an H2O2‐unreactive di‐Fe3+ form. The data obtained suggest that the primary role of EcBfr in vivo may be to detoxify H2O2 rather than sequester iron

The Role of CyaY in Iron Sulfur Cluster Assembly on the E. coli IscU Scaffold Protein

Progress in understanding the mechanism underlying the enzymatic formation of iron-sulfur clusters is difficult since it involves a complex reaction and a multi-component system. By exploiting different spectroscopies, we characterize the effect on the enzymatic kinetics of cluster formation of CyaY, the bacterial ortholog of frataxin, on cluster formation on the scaffold protein IscU. Frataxin/CyaY is a highly conserved protein implicated in an incurable ataxia in humans. Previous studies had suggested a role of CyaY as an inhibitor of iron sulfur cluster formation. Similar studies on the eukaryotic proteins have however suggested for frataxin a role as an activator. Our studies independently confirm that CyaY slows down the reaction and shed new light onto the mechanism by which CyaY works. We observe that the presence of CyaY does not alter the relative ratio between [2Fe2S]2+ and [4Fe4S]2+ but directly affects enzymatic activity

Bose-Einstein Condensation in Magnetic Insulators

The elementary excitations in antiferromagnets are magnons, quasiparticles with integer spin and Bose statistics. In an experiment their density is controlled efficiently by an applied magnetic field and can be made finite to cause the formation of a Bose-Einstein condensate (BEC). Studies of magnon condensation in a growing number of magnetic materials provide a unique window into an exciting world of quantum phase transitions (QPT) and exotic quantum states.Comment: 17 pages, 3 figure

Formation of low-valent Fe0^0 and FeI^I species in Fe-catalyzed cross-coupling chemistry: key role of ate-FeII^{II} intermediates

International audienceAte-iron(lI) species such as [Ar$_3$Fe$^{II}$-(Ar = aryl) are key intermediates in Fe-catalyzed cross-coupling reactions between aryl Grignard reagents (ArMgX) and organic electrophiles.They can be active species in the catalytic cycle, or lead to Feo and Fel oxidation states. These low oxidation states were shown to be catalytically active in some cases, but they mostly lead to unwished organic byproducts. This works relates a study of the evolution of [Ar$_3$Fe$^{II}$]-complexes towards Feo and Fel oxidation states, through $^1$H NMR, EPR and $^{57}$Fe-Môssbauer spectroscopies, as weil as DFT calculations, so as to discuss the role of both steric parameters and spin states in the reduction processes. Such mechanistic insights give a betler understanding of iron-catalyzed C-C bond formation reactions, and can be exploited in the design of new ligands in order to selectively obtain a sole iron oxidation state in a catalytic process

Iron-catalyzed C―C cross-coupling in the absence of additional ligands: active species and off-cycle pathways

International audienceIron-catalyzed cross-coupling between a Grignard reagent RMgX and an electrophile R'─X was discovered by Kochi in the 1970s and witnessed recent improvements. This transformation can be carried out using simple iron salts such as FeCl$_2$ , FeCl$_3$ or Fe(acac)$_3$ in the absence of additional ligand. However, these systems lead to short-lived reactive species, making in-situ mechanistic analysis challenging. By means of Mössbauer, cw-and pulse-EPR spectroscopies, we demonstrated that two arene-stabilized Fe$^0$ and Fe$^I$ resting states were obtained by reduction of the precursor in toluene (Fig. 1a). Analysis of the bulk revealed that the ($\eta^4$-C$_6$H$_5$Me)$_2$Fe$^0$ complex catalyzes efficiently aryl-heteroaryl coupling, via a Fe$^0$ /Fe$^{II}$ cycle (Fig. 1b). Preliminary results moreover show that transient tris(aryl) species such as [Ph$_3$ Fe$^{II}$ ]-are key intermediates in the formation of the lower oxidation states. Fe$^0$ and Fe$^I$ are respectively afforded by 2-electron reductive elimination and by redox disproportionation of the +II ox. state

Formation of low-valent Fe0^0 and FeI^I species in Fe-catalyzed cross-coupling chemistry: key role of ate-FeII^{II} intermediates

International audienceAte-iron(lI) species such as [Ar$_3$Fe$^{II}$-(Ar = aryl) are key intermediates in Fe-catalyzed cross-coupling reactions between aryl Grignard reagents (ArMgX) and organic electrophiles.They can be active species in the catalytic cycle, or lead to Feo and Fel oxidation states. These low oxidation states were shown to be catalytically active in some cases, but they mostly lead to unwished organic byproducts. This works relates a study of the evolution of [Ar$_3$Fe$^{II}$]-complexes towards Feo and Fel oxidation states, through $^1$H NMR, EPR and $^{57}$Fe-Môssbauer spectroscopies, as weil as DFT calculations, so as to discuss the role of both steric parameters and spin states in the reduction processes. Such mechanistic insights give a betler understanding of iron-catalyzed C-C bond formation reactions, and can be exploited in the design of new ligands in order to selectively obtain a sole iron oxidation state in a catalytic process

Iron-catalyzed C―C cross-coupling in the absence of additional ligands: active species and off-cycle pathways

International audienceIron-catalyzed cross-coupling between a Grignard reagent RMgX and an electrophile R'─X was discovered by Kochi in the 1970s and witnessed recent improvements. This transformation can be carried out using simple iron salts such as FeCl$_2$ , FeCl$_3$ or Fe(acac)$_3$ in the absence of additional ligand. However, these systems lead to short-lived reactive species, making in-situ mechanistic analysis challenging. By means of Mössbauer, cw-and pulse-EPR spectroscopies, we demonstrated that two arene-stabilized Fe$^0$ and Fe$^I$ resting states were obtained by reduction of the precursor in toluene (Fig. 1a). Analysis of the bulk revealed that the ($\eta^4$-C$_6$H$_5$Me)$_2$Fe$^0$ complex catalyzes efficiently aryl-heteroaryl coupling, via a Fe$^0$ /Fe$^{II}$ cycle (Fig. 1b). Preliminary results moreover show that transient tris(aryl) species such as [Ph$_3$ Fe$^{II}$ ]-are key intermediates in the formation of the lower oxidation states. Fe$^0$ and Fe$^I$ are respectively afforded by 2-electron reductive elimination and by redox disproportionation of the +II ox. state

Biologically relevant heterodinuclear iron-manganese complexes.

International audienceThe heterodinuclear complexes [Fe(III)Mn(II)(L-Bn)(μ-OAc)(2)](ClO(4))(2) (1) and [Fe(II)Mn(II)(L-Bn)(μ-OAc)(2)](ClO(4)) (2) with the unsymmetrical dinucleating ligand HL-Bn {[2-bis[(2-pyridylmethyl)aminomethyl]]-6-[benzyl-2-(pyridylmethyl)aminomethyl]-4-methylphenol} were synthesized and characterized as biologically relevant models of the new Fe/Mn class of nonheme enzymes. Crystallographic studies have been completed on compound 1 and reveal an Fe(III)Mn(II)μ-phenoxobis(μ-carboxylato) core. A single location of the Fe(III) ion in 1 and of the Fe(II) ion in 2 was demonstrated by Mössbauer and (1)H NMR spectroscopies, respectively. An investigation of the temperature dependence of the magnetic susceptibility of 1 revealed a moderate antiferromagnetic interaction (J = 20 cm(-1)) between the high-spin Fe(III) and Mn(II) ions in 1, which was confirmed by Mössbauer and electron paramagnetic resonance (EPR) studies. The electrochemical properties of complex 1 are described. A quasireversible electron transfer at -40 mV versus Ag/AgCl corresponding to the Fe(III)Mn(II)/Fe(II)Mn(II) couple appears in the cyclic voltammogram. Thorough investigations of the Mössbauer and EPR signatures of complex 2 were performed. The analysis allowed evidencing of a weak antiferromagnetic interaction (J = 5.72 cm(-1)) within the Fe(II)Mn(II) pair consistent with that deduced from magnetic susceptibility measurements (J = 6.8 cm(-1)). Owing to the similar value of the Fe(II) zero-field splitting (D(Fe) = 3.55 cm(-1)), the usual treatment within the strong exchange limit was precluded and a full analysis of the electronic structure of the ground state of complex 2 was developed. This situation is reminiscent of that found in many diiron and iron-manganese enzyme active sites