Oxidation Mechanism of Black Tea Pigment Theaflavins

<p>Theaflavins (1–4) are well characterized black tea pigments with a benzotropolone chromophore. It is reported that theaflavins are oxidized during black tea production. In this study, we investigated oxidation mechanism of theaflavin gallates (2–4) in the presence of epicatechin (5).</p><p>Enzymatic oxidation of theaflavin-3-O-gallate (2) in the presence of 5 afforded theanaphthoquinone-3'-O-gallate (7), flavanotheaflavin B (9), and theadibenzotropolone A (10). Compound 7 was produced by oxidation of benzotropolone moiety, and 9 was produced by condensation of benzotropolone moiety with 5. In contrast, enzymatic oxidation of theaflavin-3'-O-gallate (3) in similar conditions gave theadibenzotropolone F (12), which was produced by condensation of galloyl moiety with 5, and oxidation of theaflavin-3,3'-di-O-gallate (4) afforded theadibenzotropolones D (13), E (11), and theatribenzotropolone A (14). Benzotropolone moiety of theaflavins with a free hydroxyl group at C-3' (1 and 2) easily reacted with 5; however, benzotropolone moiety of theaflavins having galloyl group at C-3' (3 and 4) could not react with 5. The charge-transfer interaction is important in benzotropolone oxidation, and the galloyl group at the C-3' position interfere with the interaction. We also investigated autoxidation mechanism of 2 and 3 in pH 7 buffer. Compound 7 and theacoumarin B (15) were afforded as autoxidation products of 2. On the other hand, Autoxidation of 3 gave bistheaflavin C (17). These results revealed that the difference in the position of galloyl ester or oxidation condition afford different oxidation products of theaflavins.</p>