Effect of structural elements of tannins on their reactivity with metal ions – experimental study with QM support

Tannins are a wide group of natural polyphenols that are traditionally used for leather tanning. Nowadays, these compounds are also being researched for their antioxidative, anticarcinogenic and other health-beneficial properties. Among them is also their antibacterial activity. This is to a large extent ascribed to their ability to chelate metal ions and therefore deplete growth media of metal ions. However, studies that would enable a better understanding of these interactions are still relatively rare, mostly due to the difficulty of obtaining the pure compounds. Initially, the four compounds, namely vescalin, castalin, vescalagin and castalagin, were isolated using preparative reverse-phase C18 HPLC. Finally, the metal chelating ability of isolated compounds was examined using UV/Vis spectroscopy in conjunction with a Job plot. Because the interactions of tannins with metal ions are closely connected to the deprotonation of ellagitannins, the microscopic protolytic equilibrium of ellagitannins was examined. During the work, the DFT computations were used to get better elucidation of the obtained results on the molecular level, either for the explanation of the obtained chemical shifts or to search for the most probable binding positions of metal ions.