Laser Access to Quercetin Radicals and Their Repair by Co‐antioxidants

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(−1) and 72–75 kcal mol(−1), a much narrower interval in the case of the former than previously estimated by theoretical calculations.