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) were studied by measuring the absorbance of adenosine at 260 nm and chlorogenic acid at 328 nm spectrophotometrically. t-BuO radicals were generated by the photolysis of tert-butyl hydroperoxide (t-BuOOH) in presence of tert-butyl alcohol to scavenge OH radicals. The rates and the quantum yields (phi) of oxidation of chlorogenic acid by t-BuO 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 radicals. From competition kinetics, the rate constant of chlorogenic acid reaction with t-BuO was calculated to be 3.20 x 10(9) dm3 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 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 radicals, but also repaired adenosine radicals, formed by the reaction of adenosine with t-BuO radicals.