Jean-Louis Pierre

Iron and activated oxygen species in biology: the basic chemistry.

This paper briefly presents a critical review concerning the chemical reactions involved when superoxide or hydrogen peroxide meet iron complexes. The data commented on are required for a correct interpretation of the chemical processes which play a paramount role in the biological activation of dioxygen and arise in normal metabolism as well as in pathological processes.

Crossover from semiconductor to magnetic metal in semi-Heusler phases as a function of valence electron concentration

Experimental and theoretical investigations of intermetallic semi-Heusler compounds (CoTiSn, FeTiSb, CoTiSb, NiTiSn, CoNbSn, CoVSb, NiTiSb) and their solid solutions are presented. The physical properties of these systems are found to be mostly determined by the number of valence electrons. Resistivity experiments show that compounds with 18 valence electrons are either semiconductors (CoTiSb, NiTiSn) or semi-metals (CoNbSn). The electronic structure calculations performed on 18-valence-electron systems by the KKR method show nine valence bands below the Fermi level and a gap of order 0.4-0.9 eV. A decrease or increase of the number of valence electrons in CoTiSb, NiTiSn or CoNbSn leads in either case to a metallic state and either ferromagnetic (CoTiSn, CoVSb) or paramagnetic (FeTiSb, NiTiSb) properties. The KKR results concerning 17- and 19-valence-electron systems correspond well with experimental characteristics, except in the case of CoVSb which KKR calculations predict to be a half-metallic ferromagnet, which conflicts with experimental data. Magnetization and resistivity measurements indicate that semiconductor-metal crossovers occur together with the appearance of ferromagnetism in the and series, for x near 0.4. This behaviour is discussed in the context of the KKR-CPA results.

Chemistry for an essential biological process: the reduction of ferric iron

In biological systems, the predominant form of iron is the trivalent Fe(III) form, which is potentially not readily bioavailable because of its hydrolysis and polymerization to insoluble forms. It is also the easiest of the two predominant forms of iron to chelate selectively. In a short overview of iron chemistry, we point out some of the pitfalls using standard redox potentials, comment on the interaction of ferric complexes with hydrogen peroxide to give hydroxyl radicals and address the release of iron from ferrisiderophores. In biological systems there are two classes of ferric reductases, the soluble flavin reductases found in prokaryotes, and the membrane-bound cytochrome b-like reductases found in eukaryotes. Finally the role of dissimilatory ferric reduction in microbial respiration and biomineralization is discussed.

Ferric reductases or flavin reductases

Assimilation of iron by microorganisms requires the presence of ferric reductases which participate in the mobilization of iron from ferrisiderophores. The common structural and catalytic properties of these enzymes are described and shown to be identical to those of flavin reductases. This strongly suggests that, in general, the reduction of iron depends on reduced flavins provided by flavin reductases.

Properties on request in semi-Heusler phases

Abstract Various transport and magnetic properties of semi-Heusler phases are described, which arise from the particular crystallographic structure and narrow band phenomena. Semiconducting-to-metal cross overs are observed by varying the electron concentration. The onset of weak itinerant ferromagnetism often arises in the metallic state. Even in NiMnSb, where the strong ferromagnetism results in a half metallic state below 80 K, the magnetism evolves from Heinsenberg-like to itinerant by increasing the temperature. Giant magnetoresistance effects are observed in the semiconducting rare earth compounds NiRSb.

One electron at a time oxidations and enzymaticparadigms: from metallic to non-metallic redox centers

Biomimetic models of redox metalloenzymes may serve to elucidate structure and mechanisms of these enzymes and, moreover, new catalysts for synthesis can ensue from these models. A number of metalloenzymes utilize free radicals as a catalytic competent cofactor to promote oxidation reactions. Non metallic redox centers (free radicals) may be an advantageous alternative to metallic centers. Several biomimetic approaches are described concerning two selected enzymes, copper–zinc superoxide dismutase and galactose oxidase. The advantages of organic radical instead of metallic centers in abstracting hydrogen atoms are discussed. Paradigme: il s’agit, a partir d’un objet assez simple de decouvrir une structure qui se retrouvera dans un objet plus eleve. Dictionnaire de Philosophie (Nathan ed. 1990)

OXIDATIONS BY COPPER METALLOENZYMES AND SOME BIOMIMETIC APPROACHES

Abstract This paper illustrates the various aspects of the reactivity of the Cu(II)–Cu(I) system in biological systems, with one example of an enzymatic reaction in which Cu(II) alone is oxidizing enough to carry out the reaction (superoxide dismutase), one example in which a Cu(II)-bound peroxo intermediate is the active species (tyrosinase) and the examples of galactose oxidase and copper amine oxidases in which Cu(II) is associated with a redox active organic cofactor. In some cases, we will show some illustrations of biomimetic approaches developed in our laboratories, aimed at a better understanding of reaction mechanisms and at an original design of new catalysts with potential applications in synthetic chemistry. Some comments are given concerning the respective features of copper and iron.

Intramolecularly hydrogen-bonded versus copper(II) coordinated mono- and bis-phenoxyl radicals.

Ligands bearing two salicylidene imine moieties substituted in ortho and para positions by tert-butyl groups have been electrochemically oxidized into mono- and bis-phenoxyl radicals. The process involves an intramolecular proton coupled to electron transfer and affords a radical in which the oxygen atom is hydrogen-bonded to a protonated ammonium or iminium group. A weak intramolecular dipolar interaction exists between the two phenoxyl moieties in the bis-radical species. The copper(II) complexes of these ligands have been characterized and electrochemically oxidized. The mono-phenoxyl radical species are X-band EPR silent. The bis-phenoxyl radical species exhibits a (S= 3/2) ground state: it arises from a ferromagnetic exchange coupling between the two spins of the radicals and that of the copper(II) when the spacer is rigid enough; a flexible spacer such as ethylidene induces decomplexation of at least one phenoxyl group. Metal coordination is more efficient than hydrogen-bonding to enhance the chemical stability of the mono-phenoxyl radicals.

Synthesis and NMR study of two lipophilic iron(III) sequestering agents based on 8-hydroxyquinoline; H-bonding and conformational changes

Abstract The synthesis of two tripodal iron chelating agents based on 8-hydroxyquinoline is described. The ligands consist of tris(2-aminoethylamine) (spacer) linked in 2- or 7- position to three 8-hydroxyquinoline units (allowing the complexation of iron). NMR study of these ligands in DMSO-d6 solutions evidence intramolecular H-bond networks inducing conformational changes in relation to the protonation state of the tertiary amine.

Structural, magnetic and transport properties of NiFeχAg(1−χ) heterogeneous alloys

Abstract We carried out a comprehensive study of structural, magnetic and electrotransport properties of as-deposited and annealed (Ni 80 Fe 20 ) χ Ag( 1−χ ) heterogenous alloys prepared by sputtering. The NiFe atomic concentration was varied between 15% and 40%. These alloys consist of small magnetic particles (Ni 80 Fe 20 ) embedded in a nonmagnetic matrix (Ag). The structures of these alloys were investigated by X-ray diffraction, scanning electron microscopy and high-resolution cross-section transmission electron microscopy. The magnetic measurements were made using SQUID magnetometry and ferromagnetic resonance. Magnetoresistance was measured with a conventional four-point probe between 1.5 K and room temperature in field range 0–6T. Three contributions to the magnetoresistance of these granular alloys have been clearly identified: the spin-valve (or giant) magnetoresistance as in multilayers, scattering on magnetic fluctuations (as in any ferromagnetic metal around its magnetic ordering temperature), and anisotropic magnetoresistance. These three contributions have their own dependences on the size of the magnetic particles, on the degree of intermixing between Ni 80 Fe 20 and Ag, and on temperature. We discuss the different shapes and amplitudes of magnetoresistance versus Ni 80 Fe 20 concentration or temperature and their evolution upon annealing in terms of the relative roles of these three contributions. The magnetoresistance in multilayers (current in-plane or perpendicular to the plane) and granular alloys are also compared.