Kinetic analysis and mechanistic aspects of autoxidation of catechins
Kinetic analysis and mechanistic aspects of autoxidation of catechins
复制标题
DOI：
10.1016/s0304-4165(01)00230-6
复制
发表时间：
2002-01-15
影响因子：
3
通讯作者：
Ikeda, T
中科院分区：
文献类型：
--
作者：
Mochizuki, M;Yamazaki, S;Ikeda, T
采用基于过氧化物酶的过氧化氢（H2O2）生物电化学传感器和Clark型氧电极对儿茶素的自氧化过程进行了连续监测和动力学分析。绿色茶中的四种主要儿茶素，（-）-表儿茶素，（-）-表儿茶素没食子酸酯，（-）-表没食子儿茶素。和（-）-表没食子儿茶素没食子酸酯作为模型化合物。发现分子氧（O-2）定量还原为H2O2。超氧化物歧化酶和H+抑制自氧化的初始速率，但不依赖于缓冲能力。基于这些结果，提出了一种自氧化的机制，第一步是由O-2的儿茶素的B环的单电子氧化，产生超氧阴离子（O-2（-））和半醌自由基，部分支持的电子自旋共振测量。O-2（-）作为比O-2更强的单电子氧化剂对儿茶素起作用，并被还原为H2O2。半醌自由基是更容易氧化与O-2，比完全还原儿茶素。自氧化速率随pH值的增加而增加。这种行为可以解释为O-2（-）和半醌自由基的稳定性随pH值的增加而增加，而不是酚基的酸解离。铜离子增强自氧化;最有可能的是它作为儿茶素的初始氧化步骤的催化剂。产物亚铜离子可引发芬顿反应生成羟基自由基。另一方面，由于与儿茶素形成强复合物，硼酸根离子显著抑制自氧化。本文还讨论了自氧化的生物学意义及其效应。(C)2002 Elsevier Science B.V.保留所有权利。
A peroxidase-based bioelectrochemical sensor of hydrogen peroxide (H2O2) and a Clark-type oxygen electrode were applied to continuous monitoring and kinetic analysis of the autoxidation of catechins. Four major catechins in green tea, (-)-epicatechin, (-)epicatechin gallate, (-)-epigallocatechin. and (-)-epigallocatechin gallate, were used as model compounds. It was found that dioxygen (O-2) is quantitatively reduced to H2O2. The initial rate of autoxidation is suppressed by superoxide dismutase and H+ but is independent of buffer capacity. Based on these results, a mechanism of autoxidation is proposed; the initial step is the one-electron oxidation of the B ring of catechins by O-2 to generate a superoxide anion (O-2(-)) and a semiquinone radical, as supported in part by electron spin resonance measurements. O-2(-) works as a stronger one-electron oxidant than O-2 against catechins and is reduced to H2O2. The semiquinone radical is more susceptible to oxidation with O-2, than fully reduced catechins. The autoxidation rate increases with pH. This behavior can be interpreted in terms of the increase in the stability of O-2(-) and the semiquinone radical with increasing pH, rather than the acid dissociation of phenolic groups. Cupric ion enhances autoxidation; most probably it functions as a catalyst of the initial oxidation step of catechins. The product cuprous ion can trigger a Fenton reaction to generate hydroxyl radical. On the other hand, borate ion suppresses autoxidation drastically, due to the strong complex formation with catechins. The biological significance of autoxidation and its effectors are also discussed. (C) 2002 Elsevier Science B.V. All rights reserved.