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Kinetics and Mechanism of Protection of Thymidine from Sulphate Radical Anion by Caffeic Acid under Anoxic Conditions

M. Sudha Swaraga and M. Adinarayana

The oxidation of thymidine by SO4 •− has been followed by measuring the absorbance of thymidine at 267 nm spectrophotometrically. The rates and the quantum yields (φ) of oxidation of thymidine by sulphate radical anion have been determined in the presence of different concentrations of caffeic acid. Increase in [caffeic acid] is found to decrease the rate of oxidation of thymidine suggesting that caffeic acid acts as an efficient scavenger of SO4 •− and protects thymidine from it. Sulphate radical anion competes for thymidine as well as for caffeic acid. The quantum yields of photooxidation of thymidine have been calculated from the rates of oxidation of thymidine and the light intensity absorbed by Peroxydisulphate (PDS) at 254 nm, the wavelength at which PDS is activated to sulphate radical anion. From the results of experimentally determined quantum yields (φexptl) and the quantum yields calculated (φcal) assuming caffeic acid acting only as a scavenger of SO4 •− radicals show that φexptl values are lower than φcal values. The experimentally found quantum yield values at each caffeic acid concentration and corrected for SO4 •− scavenging by caffeic acid (φ1 ) are also found to be greater than φexptl values. These observations suggest that the thymidine radicals are repaired by caffeic acid in addition to scavenging of sulphate radical anions.