KINETICS AND MECHANISM OF PROTECTION AND REPAIR OF PYRIMIDINE NUCLEOSIDES BY CHLOROGENIC ACID FROM TERT-BUTOXYL RADICALS

G. Vijayalakshmi*, M. Adinarayana and P. Jayaprakash Rao

ABSTRACT

The rates of oxidation of chlorogenic acid (CGA) by t-BuOï‚· radicals in the presence of pyrimidine nucleosides viz., thymidine, uridine and cytidine have been studied by measuring the absorbance at 328 nm (λmax of CGA) spectrophotometrically. tert-butoxyl (t-BuOï‚·) radicals are generated by the photolysis of tert-butyl hydroperoxide (t-BuOOH) in presence of tert-butyl alcohol to scavenge ï‚·OH radicals. The rates
and the quantum yields () of oxidation of CGA by t-BuOï‚· radicals have been determined in the absence and presence of varying concentrations of pyrimidine nucleosides. An increase in the concentration of nucleosides has been found to decrease the rate of oxidation of CGA suggesting that nucleosides and CGA have competed for t-BuO ï‚·radicals. From competition kinetics, the rate constant of t-BuOï‚·radical reaction with thymidine, uridine and cytidine has been calculated to be 4.23 × 107, 6.52× 107 and 3.75×107 dm3mol-1 s-1 respectively. The quantum yields (expt) have been calculated from the experimentally determined rates of oxidation of CGA under different experimental conditions. Assuming that CGA acts as a scavenger t-BuOï‚· radicals only, the quantum yields (cal) have been theoretically calculated. expt and cal values suggest that CGA not only protects nucleosides from t-BuOï‚· radicals but also repairs nucleoside radicals.

Keywords: chlorogenic acid, pyrimidine nucleosides, t-BuO? radicals, oxidation, protection, repair.