大豆素和染料木素的光氧化自由基反应动力学

具有共轭电子结构和多酚羟基官能团的类黄酮是天然抗氧化剂，其活性位点及其自由基稳定性是影响抗氧化效能的重要因素. 我们通过时间分辨光谱并结合量化计算对比研究了大豆素和染料木素两种异黄酮的脱质子形式由光氧化引发的自由基反应动力学. 结果表明，光氧化大豆素的酚氧阴离子先产生不稳定的中间态自由基，随后通过分子内电子转移反应生成相对稳定的自由基；异黄酮染料木素的酚氧阴离子光氧化后直接生成自旋密度在整个分子骨架上离域的稳定自由基；染料木素的5位羟基起到增强4’位酚羟基抗氧化活性的作用. 这些结果解释了染料木素远高于大豆素的抗氧化活性.

Mechanistic Studies on Radical Dynamics of Daidzein and Genistein upon Photooxidation

As natural antioxidants, flavonoids, containing polyphenolic hydroxyls and conjugated electronic structures, can inhibit chain oxidation reaction by scavenging radicals. The antioxidation active sites of flavonoids and the stabilities of product radicals are the basically factors on affecting their antioxidant reactivities. This work compares the dynamics of radicals formed from photooxidation on anionic daidzein and genistein, respectively, as revealed by the use of time-resolved spectroscopies combined with quantum chemical analyses. It is found that direct photooxidation of mono- and di-anionic daidzein both formed an intermediate radical which transformed into a relatively more stable radical via intramolecular electron trasfer. Photooxidation of anionic genistein, an isoflavone with one more 5-hydroxyl than daidzein, including mono-, di- and tri-anions, results in the formation of a stable radical with electronic spin density delocalizing on the whole molecular backbone. The antioxidant reactivity of 4’-hydroxyl for mono-anion is enhanced by the presence of 5-hydroxyl, which is ascribed to the key structural factor for the higher antioxidant activity of genistein than daidzein in physiological condition.