Computational Analysis Of The Interaction Between Persimmon Tannin Characteristic Structural Units And Metal Ions,and Their Antioxidant Activity

Persimmon is a traditional fruit in my country,and its planting area and output rank among the top in the world.Persimmon tannin（PT）is a unique polyphenol in persimmon,and its repeating unit contains multiple active phenolic hydroxyl groups,which makes PT perform well in adsorption of metal ions and anti-oxidation.Since the exact structure of PT is still unknown,the study of its characteristic structural units has become a bridge to explore the function and application of PT.This paper used density functional theory.Epigallocatechin gallate（EGCG）,the characteristic structural unit with the highest content of PT,was used as the research object to explore its interaction with several metal ions and the feasibility of separating indium/iron ions.At the same time,four structural units（epicatechin gallate（ECG）,EGCG,A-type-linked ECG dimer（A-ECG dimer）,A-type-linked EGCG dimer（A-EGCG dimer））were also selected to study their structure and antioxidant activity characteristics.The specific research contents are as follows:The B3LYP-D3 method from density functional theory（DFT）was used to calculate and analyze the interactions between deprotonated EGCG and metal ions（Ag（Ⅰ）、Hg（Ⅱ）、Cu（Ⅱ）、Fe（Ⅱ）、Al（Ⅲ）and Au（Ⅲ））.The main areas of investigation were the complex configuration,Mayer bond order,natural population analysis（NPA）charge,binding energy,and reduced density gradient（RDG）function.The results of research showed that the interaction between deprotonated EGCG and Fe（Ⅱ）was mainly chelation,only weak chelation and electrostatic interactions existed between it and Ag（I）/Hg（II）.The configurations of the complexes showed that EGCG and Al（Ⅲ）/Au（Ⅲ）formed unique“luminal structure”metal complexes.There was not only electrostatic attraction,but also aromatic ring stacking between these two metal ions and D ring 4”O,5”O,which together made this kind of metal complexes more stable.The compound of Cu（Ⅱ）ion and deprotonated EGCG had chelation,electrostatic attraction,and aromatic ring stacking.The B3LYP-D3 method from density functional theory was used to study a series of EGCG-metal complexes.The interaction rules and adsorption characteristics of EGCG with indium/iron ions were investigated by theoretical calculation system,and the adsorption experiments were used to verify them.The calculation results showed that,EGCG had more stable adsorption of Fe（Ⅲ）and higher binding strength,compared with In（Ⅲ）.The bonding strength of deprotonated EGCG with indium or iron ion was better than that of undeprotonated EGCG.The adsorption of Fe（III）by EGCG was dominated by chelation and electrostatic interactions,while In（III）and EGCG were mainly adsorbed together by weak electrostatic interaction and aromatic ring stacking.So EGCG had a stronger adsorption effect on the iron ions.Furthermore,there was a significant competitive adsorption between In（III）and Fe（III）in the mixed solution.EGCG had the highest adsorption rate of Fe（III）in p H=4 solution,reaching 23.6%,which was much higher than its adsorption to In（III）.Based on both theory and experiments,this study verified the feasibility of using PT structural monomers for the separation of indium-iron ions and provided a theoretical basis for the application of PT in the selective adsorption of metal ions.Four characteristic structural units from PT have been selected to study the interrelationships between microstructure and antioxidant activity by using the M06-2X method from density functional theory.The main areas of investigation were the molecular structures,frontier molecular orbital,molecular electrostatic potential（MEP）,NPA charge of phenolic hydroxyl H atom,and bond dissociation energy（BDE）of phenolic hydroxyl.The results indicated that the key active sites were on the phenolic hydroxyl group of the B ring for ECG and EGCG monomers.The C₁₁-OH in ECG monomer was more active site,and the C₁₂-OH in EGCG monomer was more active site.The key active site of the two A-type linking dimers was likely on the C_20’phenolic hydroxyl group of D’ring.The order of antioxidant activity was:A-ECG dimer>A-EGCG dimer>ECG>EGCG.This study explored the active sites and antioxidant activity patterns of four characteristic structural units from PT,and provided a reference for the high value utilization of PT such as antioxidants.