Zhou Ailing, Sadik Omowunmi A
Department of Chemistry, State University of New York-Binghamton, P.O. Box 6000, Binghamton, New York 13902, USA.
J Agric Food Chem. 2008 Dec 24;56(24):12081-91. doi: 10.1021/jf802413v.
Quercetin, the most abundant flavonoid in dietary fruits and vegetables, acts as antioxidant or prooxidant depending on the environmental conditions. The antioxidant behavior is believed to involve initial oxidative steps with subsequent changes in the flavonoid skeleton, which ultimately alters the chemical and biological properties of these molecules. Although the mechanism is still unclear, it has been suggested to be strongly influenced by the surrounding media. This paper reports the oxidation of quercetin by air oxygen or autoxidation, bulk electrolysis, mushroom tyrosinase, and azodiisobutyronitrile (AIBN). The central aim of this study is to systematically examine how the similarities and differences of quercetin transformation can be affected by the nature of the oxidation systems. Using a range of molecular and structural characterization techniques (UV-vis, LC-MS, GC-MS, and NMR), the oxidation of quercetin was found to result in the generation of somewhat similar metabolites including depside, phenolic acids, and quercetin-solvent adducts, although the transformation process and quantities of each product depend on the type of oxidation method employed. The rate of quercetin autoxidation can be fitted to a monoexponential first-order decay with a k value of 6.45 x 10(-2) M(-1) s(-1). Comparison of quercetin oxidative products in the different systems provides a deeper insight into the underlying mechanism involved in the oxidation process. This work demonstrates that the presence of water and/or nucleophiles as well as different catalysts (tyrosinase, AIBN, or air oxygen in solution) may have very important implications for the formation of quinone with subsequent oxidative cleavage or polymerization. Moreover, the apparent first-order kinetics of autoxidation can indicate a rate-determining, one-electron oxidation of quercetin anions followed by two fast steps of radical disproportionation and solvent addition on the resulting quinone.
槲皮素是膳食水果和蔬菜中含量最丰富的类黄酮，根据环境条件的不同，它既可以作为抗氧化剂，也可以作为促氧化剂。人们认为其抗氧化行为涉及最初的氧化步骤以及随后类黄酮骨架的变化，这最终会改变这些分子的化学和生物学性质。尽管其机制仍不清楚，但有人认为它会受到周围介质的强烈影响。本文报道了槲皮素在空气氧气或自氧化、本体电解、蘑菇酪氨酸酶以及偶氮二异丁腈（AIBN）作用下的氧化过程。本研究的核心目的是系统地研究氧化体系的性质如何影响槲皮素转化的异同。通过一系列分子和结构表征技术（紫外可见光谱、液相色谱 - 质谱联用、气相色谱 - 质谱联用和核磁共振）发现，槲皮素的氧化会产生一些相似的代谢产物，包括缩酚酸、酚酸和槲皮素 - 溶剂加合物，尽管每种产物的转化过程和数量取决于所采用的氧化方法类型。槲皮素自氧化的速率可以拟合为单指数一级衰减，k值为6.45×10⁻² M⁻¹ s⁻¹。比较不同体系中槲皮素的氧化产物，可以更深入地了解氧化过程背后的机制。这项工作表明，水和/或亲核试剂的存在以及不同的催化剂（酪氨酸酶、AIBN或溶液中的空气氧气）可能对醌的形成以及随后的氧化裂解或聚合具有非常重要的影响。此外，自氧化的表观一级动力学表明，槲皮素阴离子的速率决定的单电子氧化之后是两个快速的自由基歧化和溶剂加成到所得醌上的步骤。
