FLAVONOIDS AS ANTIOXIDANTS
FLAVONOIDS AS ANTIOXIDANTS
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DOI：
10.1021/ja00090a032
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发表时间：
1994-06-01
影响因子：
15
通讯作者：
SIMIC, MG
中科院分区：
文献类型：
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作者：
JOVANOVIC, SV;STEENKEN, S;SIMIC, MG
采用脉冲辐解法研究了3，4-二羟基苯衍生物和部分黄酮类化合物的苯氧基自由基的光谱、酸碱和氧化还原性质。从苯氧基光谱的pH依赖性的变化，解离常数推导。儿茶素（pK（a）= 4.6）和芦丁（pK（a）= 4.3）自由基中3 '-OH基团的去质子化的pK（a）值与3，4-二羟基苯甲酸酯自由基的pK（a）值pK（a）= 4.2相似，这是从它们相似的电子结构中预期的。儿茶素和芦丁中的5-和7-OH以及橙皮苷中的5-OH的去质子化对自由基光谱没有影响，这是由于黄酮类自由基的A环与未配对电子的偶联效率低，由于苯氧基自由基的还原电位良好，E（7）= 0.56- 0.7Vvs NHE，类黄酮可作为烷基过氧基和超氧/氢过氧基的有效抗氧化剂。采用交流动态电导法测定了水溶液中黄酮类化合物和酚类化合物与超氧阴离子自由基反应的低反应速率常数。超氧自由基与黄酮类化合物的反应速率k = 3 × 10（2）-5.1 × 10（4）M（-1）s（-1），取决于黄酮类化合物的氧化还原性质和电荷。槲皮素和芦丁的氧化率最高，而最低的是那些B-环单取代的衍生物，具有显着较高的氧化还原电位。不带电荷的儿茶素在pH 7时的反应速率为k = 6.6 × 10（4）M（-1）s（-1），而在pH 10时，儿茶素带双重负电荷，反应速率约低4倍，k = 1.8 × 10（4）M（-1）s（-1）。确定了芦丁和trolox在pH 10下以及没食子酸甲酯在pH 7下的氧化的活化参数，以试图理解为什么尽管Δ E大于或等于0.4V的高驱动力，超氧化物反应的速率仍低。低活化熵，Δ H-双匕首= 2.3-3.6 kcal/mol，以及负活化熵，δ S-双匕首= -25-28 cal/（mol K），表明为内层电子转移机理。
Spectral, acid-base, and redox properties of the phenoxyl radicals derived from 3,4-dihydroxybenzene derivatives and selected flavonoids were studied by pulse radiolysis of aqueous solutions. From the pH-dependent changes in the phenoxyl spectra, the dissociation constants were derived. The pK(a), values for the deprotonation of the 3'-OH group in the catechin (pK(a) = 4.6) and rutin (pK(a) = 4.3) radicals are similar to the pK(a) value of the 3,4-dihydroxybenzoate radicals, pK(a) = 4.2, which is expected from their similar electronic structures. Deprotonation of 5- and 7-OH in the catechin and rutin and of 5-OH in the hesperidin radicals has no effect on the radical spectra, which is explained by the inefficient coupling of the A-ring of the flavonoid radicals with the unpaired electron. Because of favorable reduction potentials of the phenoxyl radicals, E(7) = 0.56-0.7 V vs NHE, flavonoids may act as efficient antioxidants of alkylperoxyl and superoxide/hydroperoxyl radicals. The ac kinetic conductivity method was developed for the measurements of the low reaction rate constants of the superoxide radical reactions with flavonoids and phenols in aqueous solutions at pH 10. The rates of the superoxide radical reactions with flavonoids, k = 3 X 10(2)-5.1 X 10(4) M(-1) s(-1), depend on the redox properties and the charge of the flavonoids. The highest rates are measured for the oxidation of quercetin and rutin, whereas the lowest are those for the B-ring monosubstituted derivatives, with substantially higher redox potentials. Uncharged catechin at pH 7 reacts at k = 6.6 X 10(4) M(-1) s(-1), whereas the rate at pH 10, where catechin is doubly negatively charged, is approximately 4 times lower, k = 1.8 x 10(4) M(-1) s(-1). The activation parameters of the oxidation of rutin and trolox at pH 10 and methyl gallate at pH 7 were determined in an attempt to understand why the rates of the superoxide reactions are low despite high driving forces of Delta E greater than or equal to 0.4 V. Low activation enthalpies, Delta H-double dagger = 2.3-3.6 kcal/mol, and negative activation entropies, Delta S-double dagger = -25-28 cal/(mol K), point to an inner-sphere electron-transfer mechanism.