A new synthetic approach to the ferritin core uncovers the soluble iron(III) oxo-hydroxo aggregate [Fe₁₁O₆(OH)₆(O₂CPh)₁₅]

Abstract

Hydrolytic polymerization of iron(III) occurs in many reactions in vivo, for example, the formation of bacterial magnetite in magnetotactic organisms¹, biomineralization of iron² and the synthesis of the metallic core of the iron-storage protein ferritin³. The ferritin core contains aggregates of up to 4,500 oxygen-bridged, octahedrally coordinated, high-spin iron(III) centres and is attached to the protein shell through carboxylate groups of amino-acid side chains. The X-ray and electron-diffraction patterns of this core resemble those of the mineral ferrihydrite⁴, a hydrated iron oxide formed in nature, inter alia, by iron-dependent bacteria. The preparation and structural characterization of such large poly-iron aggregates has been a challenge to inorganic chemists^5,6. We have recently shown that tri- and tetranuclear iron(III) oxo complexes of the type thought^3,7,8 to be important in ferritin-core formation can be prepared by reacting mononuclear [FeCl₄]− and binuclear [Fe₂OCl₆]²⁻components in aprotic solvents (ref. 9 and S.M.G., W. H. Armstrong and S.J.L., in preparation). Here we report the discovery of a remarkable new molecule, [Fe₁₁0₆(OH)₆(0₂CPh)₁₅], obtained by hydrolysis of the {Fe₂O}⁴⁺ unit in the presence of limited amounts of water and carboxylate salts. The synthesis and properties of this soluble iron(III) oxo-hydroxo aggregate should help to elucidate the mechanism of formation of poly-iron centres.