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

Abstract

The recognition of quinolinic carboxylic acids as natural chelants 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 the chelation properties of 8-HQA 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 FeL_qH_r species are formed, with q = 1, 2 (and 3), and −2 ≤ r ≤ 1. The presence of the main FeL_qH_r 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 pL_0.5 values, resulted in it being highly dependent on the pH value: (i) at relatively low pH values, it is higher for Fe³⁺ (pL_0.5 = 6.3 at pH = 3.0) than for Fe²⁺ (pL_0.5 = 3.1 at pH = 3.0); (ii) it is almost the same at pH = 8.1 (Fe³⁺: pL_0.5 = 8.3; Fe²⁺: pL_0.5 = 8.1); (iii) it is higher for Fe²⁺ at high pH values (pL_0.5 = 8.9 for Fe²⁺ and pL_0.5 = 6.2 for Fe³⁺ at pH = 10.0). The determination of the stability constants of the Fe²⁺/8-HQA and Fe³⁺/8-HQA complexes was also complemented by data obtained by the ligand-competition approach, using EDTA as a competing ligand over a wide range of cation and ligand concentrations and ratios. This also allowed a more thorough investigation of both the Fe²⁺/EDTA and Fe³⁺/EDTA systems, providing an accurate stability constant dataset for the Fe_p(EDTA)_qH_r species under the above-mentioned experimental conditions, which are commonly used in biological studies.