Antioxidants are molecules that can prevent the harmful effects of oxygen, help capture and neutralize free radicals, and thus eliminate the damage of free radicals to the human body. Persimmon tannin (PT) has excellent antioxidant activity, which is closely related to its molecular structure. We report here a comparative study of four characteristic structural units from PT (epicatechin gallate (ECG), epigallocatechin gallate (EGCG), A−type linked ECG dimer (A−ECG dimer), A−type linked EGCG dimer (A−EGCG dimer)) to explore the structure–activity relationship by using the density functional theory. Based on the antioxidation mechanism of hydrogen atom transfer, the most favorable active site for each molecule exerts antioxidant activity is determined. The structural parameters, molecular electrostatic potential, and frontier molecular orbital indicate that the key active sites are located on the phenolic hydroxyl group of the B ring for ECG and EGCG monomers, and the key active sites of the two dimers are located on the phenolic hydroxyl groups of the A and D’ rings. The natural bond orbital and bond dissociation energy of the phenolic hydroxyl hydrogen atom show that the C₁₁−OH in the ECG monomer and the C₁₂−OH in the EGCG monomer are the most preferential sites, respectively. The most active site of the two A−linked dimers is likely located on the D’ ring C_20′ phenolic hydroxyl group. Based on computational analysis of quantum chemical parameters, the A−ECG dimer is a more potent antioxidant than the A−EGCG dimer, ECG, and EGCG. This computational analysis provides the structure–activity relationship of the four characteristic units which will contribute to the development of the application of PT antioxidants in the future.

Hydrogen embrittlement causes deterioration of materials used in metal–hydrogen systems. Alloying is a good option for overcoming this issue. In the present work, first-principles calculations were performed to systematically study the effects of adding Ni on the stability, dissolution, trapping, and diffusion behaviour of interstitial/vacancy H atoms of pure V. The results of lattice dynamics and solution energy analyses showed that the V–Ni solid solutions are dynamically and thermodynamically stable, and adding Ni to pure V can reduce the structural stability of various VH_x phases and enhance their resistance to H embrittlement. H atoms preferentially occupy the characteristic tetrahedral interstitial site (TIS) and the octahedral interstitial site (OIS), which are composed by different metal atoms, and rapidly diffuse along both the energetically favourable TIS → TIS and OIS → OIS paths. The trapping energy of monovacancy H atoms revealed that Ni addition could help minimise the H trapping ability of the vacancies and suppress the retention of H in V. Monovacancy defects block the diffusion of H atoms more than the interstitials, as determined from the calculated H-diffusion barrier energy data, whereas Ni doping contributes negligibly toward improving the H-diffusion coefficient.

柿单宁具有优良的吸附重金属离子的效能, 表没食子儿茶素没食子酸酯(EGCG)是柿单宁发挥其活性作用的关键结构单体. 为分析柿单宁与金属离子相互作用的本质, 本文利用密度泛函理论(density functional thoery, DFT)的B3LYP方法, 从EGCG-金属复合物的构型、Mayer键级、自然电荷布居、结合能、以及弱相互作用等方面, 计算分析了EGCG与金属离子(Ag⁺, Hg²⁺, Cu²⁺, Fe²⁺, In³⁺, Al³⁺, Au³⁺)的相互作用关系. 研究结果表明, EGCG-Fe²⁺复合物主要以螯合键的形式吸附在一起; 而EGCG主要是通过静电吸引作用吸附Ag⁺和Hg²⁺离子; In³⁺, Al³⁺和Au³⁺离子与EGCG形成了独特的“腔状结构”金属复合物; Cu²⁺离子与EGCG的复合则同时存在着螯合作用、静电吸引作用和芳环堆积作用. 结合能计算显示, 金属离子所带电荷量越多, 越容易进行电荷转移, EGCG对其的静电吸引作用就越强. 这些计算分析可为探究柿单宁吸附金属离子的机理提供帮助.

Selective recovery of indium has been widely studied to improve the resource efficiency of critical metals. However, the interaction and selective adsorption mechanism of indium/iron ions with tannin-based adsorbents is still unclear and hinders further optimization of their selective adsorption performance. In this study, the epigallocatechin gallate (EGCG) monomer, which is the key functional unit of persimmon tannin, was chosen to explore the ability and mechanism of selective separation/extraction of indium from indium–iron mixture solutions. The density functional theory calculation results indicated that the deprotonated EGCG was easier to combine with indium/iron cations than those of un-deprotonated EGCG. Moreover, the interaction of the EGCG–Fe(III) complex was dominated by chelation and electrostatic interaction, while that of the EGCG–In(III) complex was controlled by electrostatic interactions and aromatic ring stacking effects. Furthermore, the calculation of binding energy verified that EGCG exhibited a stronger affinity for Fe(III) than that for In(III) and preferentially adsorbed iron ions in acidic or neutral solutions. Further experimental results were consistent with the theoretical study, which showed that the Freundlich equilibrium isotherm fit the In(III) and Fe(III) adsorption behavior very well, and the Fe(III) adsorption processes followed a pseudo-second-order model. Thermodynamics data revealed that the adsorption of In(III) and Fe(III) onto EGCG was feasible, spontaneous, and endothermic. The adsorption rate of the EGCG monomer for Fe(III) in neutral solution (1:1 mixed solution, pH = 3.0) was 45.7%, 4.3 times that of In(III) (10.7%). This study provides an in-depth understanding of the relationship between the structure of EGCG and the selective adsorption capacity at the molecular level and provides theoretical guidance for further optimization of the selective adsorption performance of structurally similar tannin-based adsorbents.