Citric acid plays an ubiquitous role in the complexation of essential metals like iron and thus it has a key function making them biologically available. For this, iron(III) citrate complexes are considered among the most significant coordinated forms of ferric iron that take place in biochemical processes of all living organisms. Although these systems hold great biological relevance, their coordination chemistry has not been fully elucidated yet. The current study aimed to investigate the speciation of iron(III) citrate using Mössbauer and electron paramagnetic resonance spectroscopies. Our aim was to gain insights into the structure and nuclearity of the complexes depending on the pH and iron to citrate ratio. By applying the frozen solution technique, the results obtained directly reflect the iron speciation present in the aqueous solution. At 1:1 iron:citrate molar ratio, polynuclear species prevailed forming most probably a trinuclear structure. In the case of citrate excess, the coexistence of several monoiron species with different coordination environments was confirmed. The stability of the polynuclear complexes was checked in the presence of organic solvents.

Department of Analytical Chemistry Institute of Chemistry ELTE Eötvös Loránd University Pázmány P s 1 A 1117 Budapest Hungary

Department of Complex Fluids Institute for Solid State Physics and Optics Wigner Research Centre for Physics Konkoly Thege Miklós út 29 33 1121 Budapest Hungary

Department of Experimental Physics Faculty of Science Palacký University Olomouc 17 listopadu 1192 12 771 46 Olomouc Czech Republic

Department of Plant Physiology and Molecular Plant Biology Institute of Biology ELTE Eötvös Loránd University Pázmány Péter s 1 C 1117 Budapest Hungary

Nuclear Analysis and Radiography Department Centre for Energy Research Konkoly Thege Miklós út 29 33 1121 Budapest Hungary

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