Frešer Franjo, Hostnik Gregor, Tošović Jelena, Bren Urban
Faculty of Chemistry and Chemical Technology, University of Maribor, Smetanova 17, SI-2000 Maribor, Slovenia.
Faculty of Mathematics, Natural Sciences and Information Technologies, University of Primorska, SI-6000 Koper, Slovenia.
Foods. 2021 Nov 3;10(11):2689. doi: 10.3390/foods10112689.
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.
单宁的一个重要特性涉及其与金属离子形成配位化合物的能力，这对于这些离子的生物利用度以及单宁的抗菌和抗氧化活性至关重要。在本研究中，使用紫外可见光谱结合密度泛函理论（DFT）计算，研究了单宁的基本组成单元之一没食子酸与Fe(II)离子之间相互作用的pH依赖性。此外，还建立了两个模型来解释溶液中发生的过程。第一个模型将反应视为简单的双分子过程，而第二个模型还考虑了质子解离平衡，事实证明该模型在识别形成常数的pH依赖性方面非常成功，其假设得到了DFT计算的有力支持。我们表明，两次去质子化的没食子酸物种与Fe(II)离子以1:1的摩尔比形成配位化合物。为了更深入地了解该过程，还使用各种DFT泛函研究了配位化合物的形成，这进一步支持了模型结果。此外，由于所需样品量相对较少，这里开发的方法将有助于研究更难分离的化合物。
