Frontier Orbitals and Аctive Site of Тea Рolyphenol Мolecules Epigallocatechin Gallate and Gallocatechin Gallate

Epigallocatechin gallate (EGCG or (2S,3R)-5,7-dihydroxy-2-(3,4,5-trihydroxyphenyl)-3,4-dihydro-2H-chromen-3-yl 3,4,5-trihydroxybenzoate) and its isomers gallocatechin gallate (GCG or (2R,3R)-2-(3,4,5-trihydroxyphenyl)-3,4-dihydro-1(2H)-benzopyran-3,5,7-triol 3-(3,4,5-trihydroxybenzoate) are the main antioxidant components in tea polyphenols and tea. In this paper, the B3LYP density functional theory (DFT) was used to optimize the EGCG and GCG molecular configuration at the 6‑311G(d,p) level of theory implemented in Gaussian software. The result is convergent and has no imaginary frequency, indicating a stable structure that reaches minimum energy value. Multiwfn wave function analysis software was used to study the frontier orbitals of EGCG and GCG. The electrophilic and nucleophilic sites of the two molecules were predicted by the contribution of each atom to the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The results show that the electrophilic reaction is mainly on No.1 benzene ring, and the main contribution is the C atom, especially the para‑C atom of the C atom connected by the two phenolic hydroxyl groups on the benzene ring. At the same time, the activation of the phenolic hydroxyl group on the para- and ortho-C atoms on the benzene ring was verified. When there is a phenolic hydroxyl group on the benzene ring, its para- and ortho-C atoms become more active, and they are easier to lose electrons. So substitution or other reactions occurs. This site is also the main site against oxygen free radicals which is an accelerated aging substance. The nucleophilic sites are mainly around C=O and no. 3 benzene rings, so it has certain ability to obtain electrons. Finally, the active sites of EGCG were analyzed by using the two methods of Fukui function and mean partial ionization energy, and the results were basically consistent with the above.