Kinetics of oxidation of adenosine by tert-butoxyl radicals: Protection and repair by chlorogenic acid

The rates of oxidation of adenosine and chlorogenic acid by tert-butoxyl radicals (t-BuO(center dot)) were studied by measuring the absorbance of adenosine at 260 nm and chlorogenic acid at 328 nm spectrophotometrically. t-BuO(center dot) radicals were generated by the photolysis of tert-butyl hydroperoxide (t-BuOOH) in presence of tert-butyl alcohol to scavenge OH(center dot) radicals. The rates and the quantum yields (phi) of oxidation of chlorogenic acid by t-BuO(center dot)radicals were determined in the absence and presence of varying concentrations of adenosine. An increase in the concentration of adenosine was found to decrease the rate of oxidation of chlorogenic acid, suggesting that adenosine and chlorogenic acid competed for t-BuO(center dot) radicals. From competition kinetics, the rate constant of chlorogenic acid reaction with t-BuO(center dot) was calculated to be 3.20 x 109 dm(3) mol(-1) s(-1): The quantum yields (phi(expt)) were calculated from the experimentally determined rates of oxidation of chlorogenic acid under different experimental conditions. Assuming that chlorogenic acid acts as a scavenger of t-BuO(center dot) radicals only, the quantum yields (phi(cal)) were theoretically calculated. phi(expt) and phi cal values suggested that chlorogenic acid not only protected adenosine from t-BuO(center dot) radicals, but also repaired adenosine radicals, formed by the reaction of adenosine with t-BuO(center dot) radicals.