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In recent years, plant polyphenols have attracted great attention due to their wide range of biological activities. Certain kinds of polyphenols have complex structures; therefore, it is difficult to elucidate their total structure, including stereochemistry. In this study, we reinvestigated the stereostructures of two major C-glycosidic ellagitannins contained in Quercus plants, vescalagin and castalagin, and revised their stereostructures based on theoretical calculations of spectroscopic data. We also determined the structures of quercusnins A and B, isolated from the sapwood of Quercus crispula, based on theoretical calculations of NMR data. The oxidation mechanism of polyphenols has not been entirely elucidated. Therefore, we have also studied the oxidation mechanism of tea catechins during black tea production. Our investigation of the oxidation mechanism of black tea pigment theafiavins revealed that the difference in the position of the galloyl ester affords different oxidation products of theafiavins. In addition, oxidation products of pyrogallol-type catechins could be classified into three types-dehydrotheasinensins, theacitrins, and proepitheafiagallins; their detailed production and degradation mechanisms were also examined., YAKUGAKU ZASSHI, 137(3), pp.347-354; 2017

Recent biological and pharmacological studies strongly suggested that plant polyphenols in foods, beverages and crude drugs have various health benefits. However, still there are chemically uncharacterized polyphenols, especially those with large molecular weights. The typical example is black tea polyphenols. Four tea catechins of fresh tea leaves are enzymatically oxidized in tea fermentation process of black tea manufacture to give a complex mixture of the oxidation products. Despite many efforts since 1950's, major part of the black tea polyphenols has not been clarified yet. We have investigated the oxidation mechanism of each catechin by employing a newly developed in vitro model fermentation system. The oxidation was initiated by enzymatic dehydrogenation of catechins, and subsequent intermolecular quinone-phenol coupling reactions followed by cascade-type degradation of the unstable products resulted in the formation of complex black tea polyphenols. Besides black tea polyphenols, this review introduces the chemistry of insolubilization of persimmon proanthocyanidins, wood polyphenols in connection with whisky polyphenols, and co-polymerization of cinnamaldehyde and proanthocyanidins in cinnamon bark., 平成19年度宮田記念学術論文賞受賞記念総説, 藥學雜誌, 128(8), pp.1119-1131; 2008

紅茶の抗酸化能をクメンヒドロペルオキシド/へモグロビン・メチレンブルー法で測定した.市販の紅茶11種類を用いた.スリランカ産のヌワラエリアがもっとも高い抗酸化能で320nmol/mlであり,中国産のキーモンがもっとも低い抗酸化能の103nmol/mlであった.これらの紅茶11種類の抗酸化能の平均は201 nmol/mlであった.紅茶の抽出液を魔法瓶で保存すると,抗酸化能は3時間で10-15%減少した.また紅茶の残留農薬試験を行ったが,調べた5種類の農薬(DDTBHC,アルドリン,ディルドリン,エンドリン)は検出されなかった.これらのことから紅茶は活性酸素を消去する安全な健康飲料と言えるので,健康維持・病気予防に活用が勧められる., Antioxidant activity in black tea was measured by the cumene hydroperoxide/haemoglobin･methylene blue method developed in our laboratory. We examined ll kinds of commercially available black tea leaves. We found the highest antioxidant activity (320 nmol/ml) in NUWAEA ELIA (Sri Lanka tea) and the lowest antioxidant activity (103 nmol/ml) in KEEMUN (Chinese tea). The average antioxidant activity of the ll kinds of black teas was 201 nmol/ml. Antioxidant activity in black tea decreased 10～15 % when the tea was stored in a vacuumbottle at about 70℃ for three hours. Residual pesticides (DDT, BHC, Aldrin, Dieldrin, Endrin) were not detected in 11 kinds of black tea leaves, indicating that black tea is a safe drink. Weconcluded that health can be maintained with black tea.