Department of Chemistry, Renmin University of China, Beijing 100872, China.
J Agric Food Chem. 2011 Sep 28;59(18):10367-72. doi: 10.1021/jf202683n. Epub 2011 Sep 1.
Regeneration of β-carotene from the β-carotene radical cation by the 4'-propylpuerarin anion (second-order rate constant=1.5×10(9) L mol(-1) s(-1) in methanol/chloroform=1:9 (v/v) solution at 25 °C as determined by laser flash photolysis) was found to be marginally slower than regeneration by the 7-propylpuerarin anion (2.3×10(9) L mol(-1) s(-1)), in agreement with the 7-propylpuerarin anion being more reducing (E'=0.56 V vs NHE) than the 4'-propylpuerarin anion (E'=1.01 V vs NHE). The potentials were calculated from E°=1.12 and 1.44 V (vs NHE) as determined by cyclic voltametry in aqueous solution and pKa=9.51 and 7.23 obtained previously for 7-propylpuerarin and 4'-propylpuerarin, respectively. The less reducing but more acidic 4'-propylpuerarin showed less antioxidant activity in liposome of pH 7.4, but more significant antioxidant synergism with β-carotene than the more reducing but less acidic 7-propylpuerarin for oxidation initiated in the liposome lipid phase. Electrostatic effects are concluded to be important in the regeneration of β-carotene from the radical cation in the water/lipid interface because approximately 50% of 4'-propylpuerarin is present as the anion, whereas only 0.5% of 7-propylpuerarin is present as the anion. In contrast, penetration of the undissociated phenolic group into the lipid phase, more significant for 7-propylpuerarin than for 4'-propylpuerarin according to the calculated water/lipid partition coefficients, becomes important for the chain-breaking action in lipid oxidation of the puerarin derivatives as models for (iso)flavonoids and their glycosides.
β-胡萝卜素自由基阳离子由 4'-丙基葛根素阴离子（在甲醇/氯仿=1:9（v/v）溶液中，25°C 下的二级速率常数=1.5×10(9) L mol(-1) s(-1)）再生，发现其速度比 7-丙基葛根素阴离子（2.3×10(9) L mol(-1) s(-1)）稍慢，这与 7-丙基葛根素阴离子比 4'-丙基葛根素阴离子更具还原能力（E'=0.56 V 相对于 NHE）相一致。这些电位是通过循环伏安法在水溶液中测定的 E°=1.12 和 1.44 V（相对于 NHE）以及先前分别为 7-丙基葛根素和 4'-丙基葛根素获得的 pKa=9.51 和 7.23 计算得出的。虽然 4'-丙基葛根素的还原能力较低，但酸性较强，在 pH 7.4 的脂质体中抗氧化活性较低，但与更具还原能力但酸性较弱的 7-丙基葛根素相比，对于在脂质体脂质相中引发的氧化，具有更显著的抗氧化协同作用。结论是，静电效应在水/脂质界面中从自由基阳离子再生β-胡萝卜素中很重要，因为大约 50%的 4'-丙基葛根素以阴离子形式存在，而只有 0.5%的 7-丙基葛根素以阴离子形式存在。相比之下，根据计算的水/脂质分配系数，未离解的酚基团渗透到脂质相中的情况对于葛根素衍生物作为（异）黄酮及其糖苷的模型的脂质氧化的链断裂作用更为重要。
