由 Sesamolin 酸水解生成Samin 之研究

Abstract

芝麻油良好的氧化安定性與芝麻木酚素有關，然而芝麻油中含量最多的兩種木酚素sesamin及sesamolin在試管試驗中卻僅呈現非常低的抗氧化性。過去文獻中發現若能將這類具有methylenedioxy結構的木酚素開環形成catechols，其抗氧化性將可大幅提升。本研究基於此概念，利用氧化劑與芝麻木酚素反應，期望能得到具有抗氧化性的反應產物。本研究第一部分先探討不同氧化劑對sesamin與sesamolin的影響，結果顯示過量的氧化劑對sesamin及sesamolin皆無作用，然而其中發現當在酸性條件下加入30% H2O2(aq)，Sesamolin對酸敏感因而降解形成sesamol和7R, 7’R-samin，其結構對照過去研究將sesamolin以酸水解產生sesamol與7R, 7’S-samin，其7’碳上氫氧基鍵結方向相反。第二部分探討7R, 7’R-samin之生成機制，首先探討加入雙氧水與否、反應的溫度、時間與酸濃度對7R, 7’S-samin與7R, 7’R-samin生成之影響，結果顯示加酸僅產生sesamol與7R, 7’S-samin，生成量隨反應溫度與時間增加呈正相關。以不同濃度之酸溶液與等體積雙氧水反應時，隨酸濃度增加，7R, 7’S-samin逐漸減少，反之7R, 7’R-samin逐漸增加。續探討酸與雙氧水添加之先後順序是否影響7R, 7’S-samin與7R, 7’R-samin之生成，結果顯示基質先加酸反應後產生sesamol與7R, 7’S-samin，此時再添加雙氧水時即產生sesamol與7R, 7’R-samin。過去文獻已知sesamolin經酸水解產生sesamol及oxonium ion之過渡體，接著經由分子內重排而形成sesamol與7R, 7’S-samin，綜合上述之結果推測7R, 7’S-samin於酸環境下會形成oxonium ion，雙氧水與oxonium ion以氫鍵形成七元環而使7’碳上鍵結方向相反。第三部分以Rancimat法評估7R, 7’R-samin對芝麻油三酸甘油酯的影響，結果顯示7R, 7’R-samin隨添加量增加會有促氧化的作用。

Sesame lignans are responsible for outstanding oxidative stability of sesame oil. However, sesamin and sesamolin, the major lignans in sesame oil, possess no antioxidant activity in vitro. Previous studies reported that antioxidant activity of lignans would be increased by converting methylenedioxy structures to catechols. On the basis of previous study, this study utilizes oxidants to react with sesamin and sesamolin, expected to obtain the resultant products with better antioxidant activity. In the first part of this study, we investigated the effect of different oxidant agents on sesamin and sesamolin. The results showed that excess oxidant agents had no effect on sesamin and sesamolin. However, when 30% H2O2(aq) was added under acidic condition, sesamolin was sensitive to acid and hydrolyzed to form sesamol and 7R, 7'R-samin. Such structure was compared with the finding in our previous study, sesamolin acid hydrolysis resulting in formation of sesamol and 7R, 7’S-samin, revealing that the binding of hydroxyl group on the C7’ was reversed. In the second part of this study, we studied on the mechanism of 7R, 7'R-samin formation. Firstly, we investigated the effect of hydrogen peroxide addition, reaction temperature, time-course study and acid concentration on the formation of 7R, 7'S-samin and 7R, 7'R-samin. The results showed that the addition of acid produced sesamol and 7R, 7’S-samin and the content of sesamol and 7R, 7’S-samin was positively correlated with the increase of reaction temperature and time-course. When different acid concentrations were added with an equal volume of hydrogen peroxide, 7R, 7'S-samin gradually decreased with increasing acid concentration, whereas 7R, 7'R-samin gradually increased. Secondly, we investigated the effect of the addition sequence of acid and hydrogen peroxide on 7R, 7’S-samin and 7R, 7’R-samin formation. The results showed that sesamol and 7R, 7’S-samin were formed after substrates have hydrolyzed with acid and 7R, 7’S-samin converted to 7R, 7’R-samin after hydrogen peroxide has reacted with reaction mixtures. Previous study has shown that sesamolin acid hydrolysis formed sesamol and an oxonium ion in transition state, followed by intramolecular rearrangement to form sesamol and 7R, 7’S-samin. Based on the results, it is speculated that 7R, 7'S-samin forms transitional oxonium ions in acid conditions and then hydrogen peroxide would be accessible to form hydrogen bonds with oxonium ion to generate a seven-membered ring structure and the bonding of C7' was opposite. In the third part of this study, we evaluated the effects of 7R, 7'R-samin on sesame oil triglyceride by Rancimat method. The results showed that 7R, 7'R-samin was pro-oxidation with the increase of the amount.