Martin-Luther-Universität Halle-Wittenberg, Institut für Chemie, Kurt-Mothes-Str. 2, 06120, Halle (Saale), Germany.
Chemistry. 2020 Dec 23;26(72):17428-17436. doi: 10.1002/chem.202001956. Epub 2020 Nov 20.
We have demonstrated the feasibility and ease of producing quercetin radicals by photoionization with a pulsed 355 nm laser. A conversion efficiency into radicals of 0.4 is routinely achieved throughout the pH range investigated (pH 2-9), and the radical generation is completed within a few ns. No precursor other than the parent compound is needed, and the ionization by-products do not interfere with the further fate of the radicals. With this generation method, we have characterized the quercetin radicals and studied the kinetics of their repairs by co-antioxidants such as ascorbate and 4-aminophenol. Bell-shaped pH dependences of the observed rate constants reflect opposite trends in the availability of the reacting protonation forms of radical and co-antioxidant and even at their maxima mask the much higher true rate constants. Kinetic isotope effects identify the repairs as proton-coupled electron transfers. An examination of which co-antioxidants are capable of repairing the quercetin radicals and which are not confines the bond dissociation energies of quercetin and its monoanion experimentally to 75-77 kcal mol and 72-75 kcal mol , a much narrower interval in the case of the former than previously estimated by theoretical calculations.
我们已经证明了通过使用脉冲 355nm 激光光解来产生槲皮素自由基的可行性和简便性。在整个研究的 pH 值范围内（pH 2-9），自由基的转化率通常达到 0.4，自由基的生成在几个纳秒内完成。除母体化合物外，不需要其他前体，离子化副产物不会干扰自由基的进一步命运。通过这种生成方法，我们已经对槲皮素自由基进行了表征，并研究了其与抗坏血酸和 4-氨基酚等共抗氧化剂的修复动力学。观察到的速率常数呈钟形 pH 依赖性，反映了反应质子化形式的自由基和共抗氧化剂的可用性的相反趋势，甚至在其最大值处掩盖了更高的真实速率常数。动力学同位素效应将修复鉴定为质子耦合电子转移。检查哪些共抗氧化剂能够修复槲皮素自由基，哪些不能，将槲皮素及其单阴离子的键离解能实验限制在 75-77 kcal mol 和 72-75 kcal mol ，这比以前通过理论计算估计的前者的间隔窄得多。
