Department of Chemistry, Renmin University of China, Beijing 100872, People's Republic of China.
J Phys Chem A. 2010 Jan 14;114(1):126-32. doi: 10.1021/jp907349x.
Electron transfer to radical cations of beta-carotene, zeaxanthin, canthaxanthin, and astaxanthin from each of the three acid/base forms of the diphenolic isoflavonoid daidzein and its C-glycoside puerarin, as studied by laser flash photolysis in homogeneous methanol/chloroform (1/9) solution, was found to depend on carotenoid structures and more significantly on the deprotonation degree of the isoflavonoids. None of the carotenoid radical cations reacted with the neutral forms of the isoflavonoids while the monoanionic and dianionic forms of the isoflavonoids regenerated the oxidized carotenoid. Electron transfer to the beta-carotene radical cation from the puerarin dianion followed second order kinetics with the rate constant at 25 degrees C k(2) = 5.5 x 10(9) M(-1) s(-1), zeaxanthin 8.5 x 10(9) M(-1) s(-1), canthaxanthin 6.5 x 10(10) M(-1) s(-1), and astaxanthin 11.1 x 10(10) M(-1) s(-1) approaching the diffusion limit and establishing a linear free energy relationship between rate constants and driving force. Comparable results found for the daidzein dianion indicate that the steric hindrance from the glucoside is not important suggesting the more reducing but less acidic 4'-OH/4'-O(-) as electron donors. On the basis of the rate constants obtained from kinetic analyses, the keto group of carotenoids is concluded to facilitate electron transfer. The driving force was estimated from oxidation potentials, as determined by cyclic-voltametry for puerarin and daidzein in aqueous solutions at varying pH conditions, which led to the standard reduction potentials E degrees = 1.13 and 1.10 V versus NHE corresponding to the uncharged puerarin and daidzein. For pH > pK(a2), the apparent potentials of both puerarin and daidzein became constants and were E degrees = 0.69 and 0.65 V, respectively. Electron transfer from isoflavonoids to the carotenoid radical cation, as formed during oxidative stress, is faster for astaxanthin than for the other carotenoids, which may relate to astaxanthins more effective antioxidative properties and in agreement with the highest electron accepting index of astaxanthin.
用激光闪光光解在均相甲醇/氯仿（1/9）溶液中研究了 β-胡萝卜素、玉米黄质、角黄素和虾青素的自由基阳离子与三种酸/碱形式的二酚异黄酮大豆苷元和其 C-糖苷葛根素的电子转移，发现电子转移取决于类胡萝卜素结构，更显著的是取决于异黄酮的去质子化程度。在中性形式的异黄酮存在下，没有一种类胡萝卜素自由基阳离子与异黄酮反应，而异黄酮的单阴离子和二阴离子形式则再生了氧化的类胡萝卜素。电子从葛根素二阴离子转移到 β-胡萝卜素自由基阳离子遵循二级动力学，在 25°C 时的速率常数 k(2)为 5.5×10(9)M(-1)s(-1)，玉米黄质为 8.5×10(9)M(-1)s(-1)，角黄素为 6.5×10(10)M(-1)s(-1)，虾青素为 11.1×10(10)M(-1)s(-1)，接近扩散极限，并在速率常数和驱动力之间建立了线性自由能关系。对大豆苷元二阴离子的类似结果表明，来自糖苷的空间位阻并不重要，表明更还原但酸性较弱的 4'-OH/4'-O(-)作为电子供体。基于动力学分析得到的速率常数，从酮基推断类胡萝卜素有利于电子转移。驱动力是根据在不同 pH 条件下在水溶液中通过循环伏安法测定的氧化电位来估计的，这导致了未带电的葛根素和大豆苷元的标准还原电位 E°=1.13 和 1.10V 相对于 NHE。对于 pH>pK(a2)，葛根素和大豆苷元的表观电位均变为常数，分别为 E°=0.69 和 0.65V。异黄酮向类胡萝卜素自由基阳离子的电子转移，如在氧化应激过程中形成的，对于虾青素比其他类胡萝卜素更快，这可能与虾青素更有效的抗氧化性质有关，并与虾青素的最高电子接受指数一致。
