Despite the intensive research on radical scavenging action of flavonoids, a systematic study of the thermochemistry for their mono-deprotonated species in aqueous solution is still missing. In this work, reaction enthalpies related to Sequential Proton-Loss Electron-Transfer (SPLET) mechanism were theoretically investigated for all mono-deprotonated forms of nine flavonoids: apigenin, luteolin, fisetin, kaempferol, quercetin, taxifolin, tricetin, tricin and cyanidin. Differences in reaction enthalpies of the first and the second deprotonation can be lower than 10 kJ mol-1, when two successive deprotonations occur in different aromatic rings of the molecule. For neutral flavonoids, thermodynamically preferred deprotonation sites are 4'-OH and 7-OH groups. In cyanidin (cation in native form), preferred second deprotonation site is 5-OH group. In the case of the formation of the preferred dianions, reaction enthalpies of the second proton loss are not affected by the structural distinctions between the flavonoids. In aqueous solution, deprotonated flavonoids show higher tendency to enter SPLET mechanism in comparison to Hydrogen Atom Transfer (HAT) or electron transfer.

中文翻译：

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去质子化黄酮类化合物的抗氧化作用：连续质子丢失电子转移的热力学

尽管对黄酮类化合物的自由基清除作用进行了深入研究，但仍缺乏对其单去质子化物质在水溶液中的热化学的系统研究。在这项工作中，从理论上研究了与顺序质子丢失电子转移 (SPLET) 机制相关的反应焓，适用于所有单去质子化形式的 9 种黄酮类化合物：芹菜素、木犀草素、漆黄素、山奈酚、槲皮素、紫杉叶素、三辛酸、三辛酸和花青素。当分子的不同芳环中发生两次连续的去质子化时，第一次和第二次去质子化的反应焓差异可以低于 10 kJ mol-1。对于中性类黄酮，热力学优选的去质子化位点是 4'-OH 和 7-OH 基团。在花青素（天然形式的阳离子）中，优选的第二去质子化位点是 5-OH 基团。在形成优选二价阴离子的情况下，第二个质子损失的反应焓不受黄酮类化合物之间结构差异的影响。在水溶液中，与氢原子转移 (HAT) 或电子转移相比，去质子化的黄酮类化合物更倾向于进入 SPLET 机制。