Abstract This study aimed at investigating the effect of L-cysteine on the Cu2+-induced oxidation of quercetin, within a pH range varying from 6.7 to 8.3 and temperatures varying from 53 to 87 °C. Initial examinations showed that quercetin degradation obeyed apparent first-order kinetics and it was significantly temperature-dependent. Modelling of the effect of L-cysteine by implementing response surface methodology suggested that L-cysteine did not inhibit quercetin oxidation, but it acted as an oxidation stimulant. Liquid chromatography-diode array-mass spectrometry analyses revealed the presence of typical quercetin degradation and oxidation products, including protocatechuic acid, 2-(hydroxybenzoyl)-2-hydroxybenzofuran-3(2H)-one and a quinone methide. However, the presence of two tentative quercetin/L-cysteine adducts evidenced the implication of L-cysteinyl radicals in the relevant reactions. It was concluded that the formation of L-cysteinyl radicals may promote quercetin oxidation, giving rise to substances with unknown biological significance. This fact merits further attention to illuminate possible beneficial or adverse nutritional consequences of such reactions in foods.

中文翻译：

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L-半胱氨酸对 Cu(II) 诱导的槲皮素氧化作用的动力学和建模

摘要 本研究旨在研究 L-半胱氨酸在 6.7 至 8.3 的 pH 范围和 53 至 87 °C 的温度范围内对 Cu2+ 诱导的槲皮素氧化的影响。初步检查表明，槲皮素降解遵循明显的一级动力学，并且显着依赖于温度。通过实施响应面方法对 L-半胱氨酸的影响进行建模表明 L-半胱氨酸不抑制槲皮素氧化，但它充当氧化刺激剂。液相色谱-二极管阵列-质谱分析表明存在典型的槲皮素降解和氧化产物，包括原儿茶酸、2-(羟基苯甲酰基)-2-羟基苯并呋喃-3(2H)-one 和醌甲基化物。然而，两种暂定的槲皮素/L-半胱氨酸加合物的存在证明了 L-半胱氨酸自由基在相关反应中的作用。结论是L-半胱氨酰自由基的形成可能促进槲皮素氧化，产生具有未知生物学意义的物质。这一事实值得进一步关注，以阐明食物中此类反应可能产生的有益或不利的营养后果。