Dependence of the Fe(II)-Gallic Acid Coordination Compound Formation Constant on the pH

One important property of tannins involves their ability to form coordination compounds with metal ions, which is vital for the bioavailability of these ions, as well as for the antibacterial and antioxidative activities of tannins. In this study, the pH dependence of interactions between gallic aci...

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Bibliographic Details
Published in:Foods 2021-11, Vol.10 (11), p.2689
Main Authors: Frešer, Franjo, Hostnik, Gregor, Tošović, Jelena, Bren, Urban
Format: Article
Language:English
Subjects:
Acids
Bioavailability
Coordination compounds
Density functional theory
Equilibrium
Fe(II) ions
Food science
Gallic acid
Iron
Job diagrams
Ligands
Metal ions
modeling
pH effects
Polyphenols
Spectroscopy
Spectrum analysis
Tannins
Ultraviolet spectroscopy
UV/Vis spectra
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Summary:One important property of tannins involves their ability to form coordination compounds with metal ions, which is vital for the bioavailability of these ions, as well as for the antibacterial and antioxidative activities of tannins. In this study, the pH dependence of interactions between gallic acid, one of the basic building blocks of tannins, and Fe(II) ions, was investigated using UV/Vis spectroscopy, in conjunction with density functional theory (DFT) calculations. Moreover, two models were developed to explain the processes taking place in the solution. The first model treated the reaction as a simple bimolecular process while the second also considered the protolytic equilibrium, which was proven very successful in discerning the pH dependence of formation constants, and whose assumptions were well supported by DFT calculations. We showed that the two-time deprotonated gallic acid species forms the coordination compound with Fe(II) ions in a 1:1 molar ratio. To gain better insight into the process, the coordination compound formation was also studied using various DFT functionals, which further supported the model results. Furthermore, due to the relatively low sample amounts needed, the methodology developed here will be useful to study compounds that are more difficult to isolate.
ISSN:2304-8158
2304-8158
DOI:10.3390/foods10112689