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Thermodynamic study on 8-hydroxyquinoline-2-carboxylic acid as a chelating agent for iron found in the gut of Noctuid larvae


The recognition of quinolinic carboxilic acids as natural chelant and the recent observation of a high production of 8-hydroxyquinoline-2-carboxylic acid (8-HQA) in the gut of several Noctuid larvae (e.g. Spodoptera littoralis), has inspired the study of 8-HQA chelation properties towards Fe2+ and Fe3+. Here we report a detailed characterization of the thermodynamic solution behaviour of Fe2+/8-HQA and Fe3+/8-HQA systems as a function of the pH value. The acid-base properties of 8-HQA and its binding ability towards Fe2+ and Fe3+ have been investigated over a wide range of pH values (2.0 ≤ pH ≤ 11.0) by ISE-H+ (glass electrode) potentiometric titrations in KCl(aq) at I = 0.2 mol dm-3 and at T = 298.15 K. For both oxidation states, various FeLqHr species are formed, with q = 1, 2 (and 3), and −2 ≤ r ≤ 1. The presence of the main FeLqHr species was confirmed by HESI-HRMS. ESR measurements have also been performed to get some extra information on the Fe3+ coordination, indicating a distorted octahedral symmetry around the metal center. Quantum mechanical calculations have been carried out in order to characterize the structural features of selected metal complexes. The complexing ability of 8-HQA is generally much higher for Fe3+ than Fe2+. Nevertheless, the sequestering ability of 8-HQA towards these two oxidation states of this metal ion, obtained by the calculation of several pL0.5 values, resulted to be highly dependent on the pH value: (i) at relatively low pH values it is higher for Fe3+ (pL0.5 = 6.3 at pH = 3.0) than Fe2+ (pL0.5 = 3.1 at pH = 3.0); (ii) almost similar at pH = 8.1 (Fe3+: pL0.5 = 8.3; Fe2+: pL0.5 = 8.1); (iii) higher for Fe2+ at high pH values (pL0.5 = 8.9 for Fe2+ and pL0.5 = 6.2 for Fe3+ at pH = 10.0). The determination of the stability constants of Fe2+/8-HQA and Fe3+/8-HQA complexes was also complemented by data obtained by the ligand-competition approach, using EDTA as competing ligand in a wide range of cations and ligands concentrations and ratios. This also allowed a more thorough investigation of both the Fe2+/EDTA and Fe3+/EDTA systems, providing an accurate stability constants dataset for the Fep(EDTA)qHr species under the above-mentioned experimental conditions, which are commonly used in biological studies.

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Publication details

The article was received on 10 Dec 2017, accepted on 06 Apr 2018 and first published on 09 Apr 2018

Article type: Paper
DOI: 10.1039/C7NJ04889K
Citation: New J. Chem., 2018, Accepted Manuscript
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    Thermodynamic study on 8-hydroxyquinoline-2-carboxylic acid as a chelating agent for iron found in the gut of Noctuid larvae

    S. Gama, M. Frontauria, N. Ueberschaar, G. Brancato, D. Milea, S. Sammartano and W. Plass, New J. Chem., 2018, Accepted Manuscript , DOI: 10.1039/C7NJ04889K

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