P-21 紅茶色素形成に関わるエピガロカテキン由来ビシクロ[3.2.1]オクタン骨格を持つ鍵中間体の生成分解機構(ポスター発表の部)

In order to clarify the chemical mechanisms of catechin oxidation during black tea production, in vitro enzymatic oxidation of epigallocatechin 3-O-gallate was examined in detail, and production mechanism of theacitrin C, an unstable black tea pigment, was confirmed. In addition, a new catechin dimer produced by decomposition of theacitrins was isolated from commercial black tea. The results suggested that the theacitrins are precursor of minor black tea polyphenols. Furthermore, it was demonstrated that the C-3 hydroxyl group of epigallocatechin plays an important role in the production of theacitrins and proepitheaflagallin. Enzymatic oxidation of epigallocatechin 3-O-acetate yielded three products, the composition of which was different from those obtained by oxidation of epigallocatechin. The difference was caused by formation of the hemiacetal structures between the free C-3 hydroxyl group and carbonyl groups generated at the B-rings. On enzymatic oxidation of epigallocatechin, we succeeded to isolate an unstable dimeric product having bicyclo[3.2.1]octane skeleton. The bicyclooctane structure was stabilized by hemiacetal formation between its carbonyl group and the C-3 hydroxyl group. Based on the HPLC analysis of the reaction mixture, the product was presumed to be a precursor of proepitheaflagallin. On the other hand, when the C-3 hydroxyl group was acylated, the bicyclo[3.2.1]octane precursor was decomposed to give theacitrins. The results indicated that the production and decomposition of bicyclo[3.2.1]octane-type intermediates are key reactions in the catechin oxidation.