Consumption of black tea contributed to many health benefits including the prevention of heart disease and certain types of cancer. However, the chemical composition of black tea has not been fully explored. Most studies have examined different interactions between the four major tea catechins, and limited studies have investigated the interaction between catechins and other components in tea. In the present study, we tested our hypothesis that the ortho-dihydroxyl structure of chlorogenic acid (CGA) could react with the vic-trihydroxy structure of (-)-epigallocatechin 3-gallate (EGCG) and (-)-epigallocatechin (EGC) to generate theaflavin-type of compounds during black tea fermentation. The reaction between CGA and EGCG or EGC was catalyzed by horseradish peroxidase (POD) in the presence of H2O2. Two theaflavin-type compounds EGCG-CGA and EGC-CGA were purified using a Sephadex LH-20 column. Their structures were elucidated on the basis of the analysis of their MS and 1D- and 2D-NMR spectroscopic data. Furthermore, the existence of these two novel compounds was characterized by LC/MS/MS analysis. We also found that EGCG-CGA and EGC-CGA had very similar inhibitory effects on the growth of human colon cancer cells with that of theaflavin 3,3'-digallate. These findings shed light on the interactions between the major bioactive compounds, catechins, and other minor compounds in tea. The confirmation of the presence of this type of reaction in black tea may provide more understanding of the complexity of black tea chemistry.
Keywords: EGC; EGCG; benzotropolone; chlorogenic acid; enzymatic model reaction.