Antioxidant properties of mono- and dihydroxyphenolic acids and their alkyl esters were examined, with emphasis on the relationship between their molecular structure and antioxidant activity. Test media with different tocopherol level were used for determining the oxidative stability: original refined sunflower oil (total tocopherols 149.0 mg/kg), partially tocopherol-stripped sunflower oil (total tocopherols 8.7 mg/kg) and distilled fatty acid methyl esters (FAME) as a tocopherol-free medium. The chemical reaction of tocopherols with diazomethane tested for the purpose to eliminate their antioxidant activity failed due to the negligible degree of methylation of hydroxyl group in the tocopherol molecule. Caffeic acid and protocatechuic acid (3,4-dihydroxyphenolic acids) and their alkyl esters were found to be more active antioxidants than monohydroxyphenolic acid (p-hydroxybenzoic acid), 2,5-dihydroxyphenolic acid (gentisic acid), 3-methoxy-4-hydroxyphenolic acids (vanillic and ferulic acids) and their corresponding alkyl esters. Naturally present tocopherols in refined sunflower oil proved to have a synergistic effect on gentisic acid but not on its alkyl esters. In contrast, tocopherols showed an antagonistic effect on alkyl esters of caffeic acid, because their protection factors decreased with increasing level of tocopherols in the test medium. Moreover, the antioxidant activity of these alkyl esters decreased with increasing length of their alkyl chain in conformity with the polar paradox hypothesis. Practical applications: Tocopherols as naturally present antioxidants influence considerably the antioxidant activity of other antioxidants added to plant oils used as a test medium. Distilled fatty acid methyl esters prepared from refined sunflower oil may serve as an optimal tocopherol-free test medium. Some alkyl esters of phenolic acids were evaluated to be applicable as natural more lipophilic antioxidants in comparison with phenolic acids.

中文翻译：

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葵花籽油中单羟基酚和二羟基酚与不同水平的天然生育酚的抗氧化作用


研究了单羟基酚酸和二羟基酚酸及其烷基酯的抗氧化特性，重点是它们的分子结构和抗氧化活性之间的关系。使用具有不同生育酚水平的测试介质来测定氧化稳定性：原始精炼葵花籽油（总生育酚 149.0 mg/kg）、部分生育酚剥离的葵花籽油（总生育酚 8.7 mg/kg）和蒸馏脂肪酸甲酯（FAME）作为不含生育酚的培养基。为消除其抗氧化活性而测试的生育酚与重氮甲烷的化学反应失败了，因为生育酚分子中羟基的甲基化程度可以忽略不计。咖啡酸和原儿茶酸（3,4-二羟基酚酸）及其烷基酯被发现是比单羟基酚酸（对羟基苯甲酸）、2,5-二羟基酚酸（龙胆酸）、3-甲氧基-4 更活跃的抗氧化剂-羟基酚酸（香草酸和阿魏酸）及其相应的烷基酯。精炼葵花油中天然存在的生育酚被证明对龙胆酸具有协同作用，但对其烷基酯没有协同作用。相反，生育酚对咖啡酸烷基酯表现出拮抗作用，因为它们的保护因子随着测试培养基中生育酚水平的增加而降低。此外，这些烷基酯的抗氧化活性随着烷基链长度的增加而降低，这符合极地悖论假说。实际应用：生育酚作为天然存在的抗氧化剂，对添加到用作测试介质的植物油中的其他抗氧化剂的抗氧化活性有很大影响。 由精炼葵花籽油制备的蒸馏脂肪酸甲酯可以作为最佳的不含生育酚的测试介质。与酚酸相比，一些酚酸烷基酯被评估为可用作天然的亲脂性更强的抗氧化剂。