Kinetics and mechanism of protection of thymine from sulphate radical anion under anoxic conditions

The rates of photooxidation of thymine in presence of peroxydisulphate (PDS) have been determined by measuring the absorbance of thymine at 264 nm spectrophotometrically. The rates and the quantum yields (φ) of oxidation of thymine 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 thymine suggesting that caffeic acid acts as an efficient scavenger of SO ₄ ^•- and protects thymine from it. Sulphate radical anion competes for thymine as well as for caffeic acid. The rate constant of sulphate radical anion with caffeic acid has been calculated to be 1.24 x 10¹⁰ dm³ mol^-1s^-1. The quantum yields of photooxidation of thymine have been calculated from the rates of oxidation of thymine and the light intensity absorbed by PDS at 254 nm, the wavelength at which PDS is activated to sulphate radical anion. From the results of experimentally determined quantum yields (φ_expt1) and the quantum yields calculated (φ_cl) assuming caffeic acid acting only as a scavenger of SO ₄ ^•- radicals show that φ_expt1 values are lower than φ_cl values. The φ ’ values, which are experimentally found quantum yield values at each caffeic acid concentration and corrected for SO ₄ ^•- scavenging by caffeic acid, are also found to be greater than φ_expt1 values. These observations suggest that the thymine radicals are repaired by caffeic acid in addition to scavenging of sulphate radical anions.