Oxidative Dimerization Mechanism of Pyrogallol-type Catechins on the Production of Black Tea Polyphenols

<p>In the process of black tea production, various oxidation products are produced from tea catechins (flavan-3-ols) by endogenous enzymes, polyphenol oxidase and peroxidase. Enzymatic oxidation of epigallocatechin-3-O-gallate (2), a major tea catechin, affords various dimers, such as dehydrotheasinensin A (3), theacitrin C (4), EGCg quinone dimer A (5), and EGCg quinone dimer B (6). In this study, we found that an unstable dimer 7was produced in the early stage of oxidation of 2. We examined the structure and reactivity of 7. We also studied the absolute configuration of 4 by using computational methods.</p><p>Epigallocatechin-3-O-gallate (2) was oxidized by polyphenol oxidase for 10 min, and the products were separated by column chromatography to afford an unstable dimer 7, along with 3–6. Compound 7 was unstable in water, and was easily degraded to give 4–6. In the early stage of oxidation of 2, compounds 3 and 7 were produced via o-quinone form of 2; however, unstable 7 gradually degraded to generate stable dimers 4–6.</p><p>Stereostructure of 3a,8a-dihydrocyclopenta[a]indene-1,6,8-trione moiety of theacitrin C (4) had not been assigned. To determine the configurations of C-3a'' and C-8a'' in 4, we calculated the ECD spectrum of theacitrinin A (12) that was produced from 4 by heating. Calculated ECD spectrum of (3a'S,8a'S)-12 agreed with experimental spectrum of 12. Next, calculation of NMR chemical shifts of (3a''S,3b''S,8a''S)-4 and (3a''S,3b''R,8a''S)-4 were performed. Calculated NMR chemical shifts of (3a''S,3b''S,8a''S)-4 were in good agreement with experimental data; therefore, the stereostructure of 4 was determined as (3a''S,3b''S,8a''S).</p>