Kinetics of Oxidation of thiamine by phosphate radical anions protection by caffecic acid

The oxidation of thymidine by PO₄^•− has been followed by measuring the absorbance of thymiine at 264 nm to 310 nm spectrophotometrically. The rates and the quantum yields (φ) of oxidation of thymine by phosphate 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 PO₄^•− and protects thymine from it. Phosphate radical anion competes for thymine as well as for caffeic acid. The quantum yields of photooxidation of thymine have been calculated from the rates of oxidation of thymidine and the light intensity absorbed by Peroxodiphosphate (PDP) at 264 nm, the wavelength at which PDP 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 PO₄^•− radicals show that φexptl values are lower than φcal values. The experimentally found quantum yield values at each caffeic acid concentration and corrected for PO₄^•− scavenging by caffeic acid (φ1) are also found to be greater than φexptl values. These observations suggest that the thymine radicals are repaired by caffeic acid in addition to scavenging of phosphate radical anions.