Calculation and analysis of interaction between characteristic functional group of persimmon tannin and metal ions

Persimmon tannin has excellent adsorption efficiency of heavy metal ions, and epigallocatechin gallate (EGCG) is the key structural monomer of persimmon tannin to play its active role. In order to analyze the nature of the interaction between persimmon tannin and metal ions, in this paper the density functional theory (DFT) is used to calculate and analyze the interactions between EGCG and metal ions (Ag⁺, Hg²⁺, Cu²⁺, Fe²⁺, In³⁺, Al³⁺, Au³⁺), from the respects of EGCG-metal complex configuration, Mayer bond order, natural population analysis, binding energy, and weak interaction. In this paper, the B3LYP combined with DFT-D3 dispersion correction method is mainly used. For metal atoms, the Lanl2dz basis set is adopted. For H, C and O atoms, the 6-311G (d, p) basis set is adopted for optimizing the structure, and the more accurate 6-311+G (d, p) basis set is selected for calculating the single point energy. At the same time, the study adds the SMD solvation model with water as the solvent. All calculations are done by using the Gaussian 09 package. The method of reduced density gradient function is used to study the weak interactions between EGCG and metal ions. The results of research show that EGCG-Fe²⁺ complex is adsorbed mainly by chelating bond. However, the EGCG adsorbs mainly Ag⁺, Hg²⁺ ions through electrostatic attraction. The configurations of the complexes show that In³⁺, Al³⁺ and Au³⁺ ions with EGCG form unique “luminal structure” metal complexes, so there is not only electrostatic attraction, but also aromatic ring stacking between these three metal ions and D ring 4<i>"</i>O, 5<i>"</i>O. The calculated Mayer bond order indicates that the bond order of the composite bond is formed by Fe²⁺ ion and the EGCG is the largest in the seven metal complexes, and the bond order is formed by In³⁺ ion, and EGCG is smallest. The compound of Cu²⁺ ion and EGCG have chelation, electrostatic attraction and aromatic ring stacking. By observing the binding energy, it can be found that the more charges the metal ions have, the easier the charge transfer will be and the stronger the electrostatic attraction of EGCG may be. These results will provide enlightenment for further studying the mechanism of persimmon tannin's adsorption of metal ions.