基于密度泛函的茶多酚分子EGCG和GCG的光谱计算

doi:10.3964/j.issn.1000-0593(2019)06-1846-06

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Abstract 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.

Key words：Epigallocatechin-3-gallate; Density functional theory; Infrared spectrum; Ultraviolet spectrum

Received: 2018-08-29 Accepted: 2019-01-18

ZTFLH:

O657.3

Corresponding Authors: TANG Yan-lin E-mail: tylgzu@163.com

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https://www.gpxygpfx.com/EN/10.3964/j.issn.1000-0593(2019)06-1846-06 OR https://www.gpxygpfx.com/EN/Y2019/V39/I06/1846