茶多酚是绿茶中主要生化活性成分之一。 选取茶多酚中含量较高, 同时也是性质较活泼、 功效较明显的表没食子儿茶素没食子酸酯(EGCG)及其异构体没食子儿茶素没食子酸酯(GCG)分子做红外光谱和紫外光谱的计算和研究。 使用Gaussian软件, 采用B3LYP密度泛函理论(DFT)在6-311g(d,p)基组水平上优化其几何构型。 频率计算得到红外光谱后, 再进行振动特征分析, 可以看到在EGCG和GCG的红外光谱图中每个振动模式下所有基团振动的权重, 结合谱图做出相应的振动归属和对比分析。 发现: 两分子红外谱图相似, 分别在1 711和1 717 cm^-1处为羰基的伸缩振动吸收峰, 苯环上酚羟基的伸缩振动吸收峰集中在3 500~3 800 cm^-1, 1 000~1 600 cm^-1的多个峰都有苯环面内弯曲振动参与, 在1 350和1 280 cm^-1附近吸收峰是亚甲基次甲基面内弯曲振动引起的, 在500 cm^-1以下吸收峰都为原子的面外弯曲振动。 采用固相粉末压片法, 使用IRPRESTIGE-21红外光谱仪测量了EGCG分子的红外光谱(400~4 000 cm^-1), 对比理论计算的EGCG分子红外光谱各吸收峰位值, 发现在固相中实际测得的EGCG分子的红外光谱与气相下的理论计算值基本吻合, 理论计算值略微有些红移, 原因可能是理论计算在气相条件下采用的势函数存在误差, 相比于无分子相互作用力的气相, 实际测量固相光谱的分子键强度比气相条件下要略大些。 使用Gaussian软件, 采用含时密度泛函理论(TD-DFT), 选取乙醇作为溶剂, 计算了EGCG分子的15个激发态, 分析了激发态的组成和能级跃迁情况。 计算所得的2个吸收峰分别位于229.3和276.4 nm处, 主要对应p电子与苯环π键上电子形成的p-π共轭的电子跃迁及苯环、 杂环上π→π^*跃迁。 从分析振子强度得知, 基态跃迁到S4, S5, S6和S12激发态为产生紫外光谱的主要原因, 另外的激发态可能为禁阻跃迁, 振子强度均小于0.01。 上述计算值与使用UV-6100S型紫外分光光度计所测得的EGCG分子在乙醇溶剂中235.1和278.7 nm的最大吸收峰吻合, 计算值略有蓝移, 可能是茶多酚提取时或本身就带有弱碱性所致。 该研究可为研究EGCG分子和GCG分子的性质和生物活性及茶多酚的抗氧化性提供理论参考。

Tea polyphenol is one of the main biochemical active components in green tea. Being a higher content of components and more active and effective ingredient in tea polyphenols, the epigallocatechin-3-gallate (EGCG) molecule and its stereoisomer GCG are selected to calculate and study the infrared spectrum and ultraviolet spectrum in this paper. In Gaussian software 09, the B3LYP density functional theory (DFT) was used to optimize the molecular geometric configuration at the base ground level of 6-311G (d,p). After frequency calculation, the infrared spectrum was obtained, and then the vibration characteristics were analyzed. It can be seen that the weight of all groups’ vibration in each vibration mode in the infrared spectrum of EGCG and GCG, and the corresponding vibration attribution and comparative analysis were made. It is found that the infrared spectra of molecule EGCG and GCG are similar. The absorption peaks for carbonyl stretching vibration are at 1 711 and 1 717 cm^-1, respectively. The absorption peak of stretching vibration of phenolic hydroxyl groups on the benzene ring is mainly concentrated in 3 500~3 800 cm^-1. Multiple absorption peaks in 1 000~1 600 cm^-1 are in-plane bending vibration of benzene. The absorption peaks near 1 350 and 1 280 cm^-1 are caused by methylene and methine in-plane bending vibration. The absorption peaks of out-plane bending vibrations are all below 500 cm^-1. The infrared spectrum of EGCG molecule (400~4 000 cm^-1) was measured through solid powder tableting method by using the IRPRESTIGE-21 infrared spectrometer manufactured by Shimadzu Corporation of Japan. The experimental infrared spectrum of EGCG is compared with the theoretical infrared spectrum. The result shows that the IR spectrum measured in the solid phase is almost consistent with the values calculated in gas phase. The theoretical infrared spectrum has slightly red-shift. The reason may be that the potential function used in theoretical calculation under the gas phase exist error. Compared with the gas phase without molecular interaction, the actual bond strength in the solid phase is slightly higher than that under the gas phase condition. In Gaussian software 09, the time-dependent density functional theory (TD-DFT) was used to calculate 15 excited states of EGCG molecules in ethanol solvent. The composition and energy level transition of the excited state were analyzed. The two absorption peaks by theoretical calculation were 229.3 and 276.4 nm, respectively. They were main corresponding the transition of p-π conjugated electron of p-electron and benzene ring π bond and the π-π^* transition on benzene ring and heterocyclic ring. According to the analysis of the intensity of the oscillator, the transition from the ground state to S4, S5, S6 and S12 excited states is the main reason for the ultraviolet spectrum. The other excited state may be the forbidden transition, because the intensity of the oscillators are all less than 0.01. The above calculated value is almost consistent with the maximum absorption peak of the experimental values of EGCG. The absorption peak of experiment is at 235.1 and 278.7 nm in ethanol solvent. The calculated value is slightly blue-shift, which may be caused by the weak alkaline of the tea polyphenols or the weak alkaline of the molecules themselves. This study can provide theoretical reference for studying the properties and biological activities of EGCG and GCG molecules and the antioxidant properties of tea polyphenols.