Radicals from the flavonoids quercetin, (+)-catechin, (+/-)-taxifolin and luteolin, and from all-rac-alpha-tocopherol have been generated electrochemically by one-electron oxidation in deaerated dimethylformamide (DMF), and characterised by electron spin resonance spectroscopy (ESR) after spin-trapping by 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). Simulations of the ESR spectrum based on estimated coupling constants of the spin-trapped quercetin radical, confirmed that this antioxidant radical is oxygen-centered. The complex mixture of radicals, quinoid intermediates and stable two-electron oxidation products, were for each antioxidant allowed to react with each of the four other antioxidants, and the progression of reaction followed by ESR after addition of DMPO, and the product solution further analysed by HPLC. All-rac-alpha-tocopherol was found to be most efficient in regenerating each of the other antioxidants from their oxidation products with a regeneration index (defined as moles regenerated of the oxidised phenolic antioxidant divided with moles of all-rac-alpha-tocopherol consumed) of 0.90+/-0.16 for quercetin, 0.48+/-0.11 for (+)-catechin, 0.48+/-0.06 for (+/-)-taxifolin and 0.50+/-0.10 for luteolin in equimolar 1.00 mM solution. Quercetin was found to have the highest regeneration index among the flavonoids: 0.88+/-0.13 for (+/-)-catechin, 0.41+/-0.03 for (+/-)-taxifolin and 0.41+/-0.02 for luteolin. The antioxidant hierarchy based on the reduction potentials determined by cyclic voltammetry under similar conditions (0.93 V for all-rac-alpha-tocopherol, 1.07 V for quercetin, 1.15 V for luteolin, 1.16V for (+)-catechin and 1.20 V for (+/-)-taxifolin) is compared with the observed over-all regeneration (34% for quercetin, 34% for (+)-catechin, 52% for (+/-)-taxifolin and 43% for luteolin by all-rac-alpha-tocopherol).

译文

黄酮类化合物槲皮素,(+)-儿茶素,(+/-)-紫杉素和木犀草素以及全rac-α-生育酚的自由基是通过脱气二甲基甲酰胺 (DMF) 中的单电子氧化而产生的,并以5,5-二甲基-1-吡咯啉-n-氧化物 (DMPO) 自旋俘获后的电子自旋共振谱 (ESR) 为特征。基于自旋捕获的槲皮素自由基的估计耦合常数的ESR光谱模拟,证实了该抗氧化剂自由基是以氧为中心的。自由基,醌中间体和稳定的两电子氧化产物的复杂混合物,使每种抗氧化剂与其他四种抗氧化剂中的每一种反应,并在反应过程中加入DMPO后进行ESR,并将产物溶液进一步通过HPLC分析。发现全rac-α-生育酚最有效地从其氧化产物中再生其他抗氧化剂,其再生指数 (定义为氧化的酚类抗氧化剂再生的摩尔数与消耗的全rac-α-生育酚的摩尔数) 为槲皮素的0.90 +/-0.16,在等摩尔1.00 mM溶液中,()-儿茶素的0.48/-0.11,(/-)-紫杉素的0.48/-0.06和木犀草素的0.50/-0.10。发现槲皮素在类黄酮中具有最高的再生指数: (/-)-儿茶素为0.88/-0.13,(/-)-紫杉素为0.41/-0.03,木犀草素为0.41/-0.02。基于在相似条件下通过循环伏安法确定的还原电位的抗氧化剂等级 (全rac-α-生育酚为0.93 V,槲皮素为1.07 V,木犀草素为1.15 V,(+)-儿茶素1.16V,(+/-)-紫杉素1.20 V) 与观察到的整体再生 (槲皮素34%,(+)-儿茶素34%,(+/-)-紫杉素的52% 和木犀草素的43%)。

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