Phosphate radical induced oxidation of thymine in aqueous solution

The photooxidation of thymine in the presence of peroxydiphosphate (PDP) in aqueous solution at natural pH (-7.5) under anoxic conditions has been carried out in a quantum yield reactor using a high pressure mercury lamp. The reactions have been followed spectrophotometrically by measuring the absorbance of thymine at lambda (264nm) and the rates calculated under different experimental conditions. The quantum yields have been calculated from the rates of reactions of oxidation of thymine and the calculated light intensity absorbed by PDP at 254nm. The intensity has been measured using the peroxydisulphate (PDS) solution as a standard chemical actinometer. On the basis of experimental results, product analysis and existence of isosbestic point, a probable mechanism is suggested in which peroxydiphosphate anion on photolysis gives phosphate radicals which initiate the reaction by adding to C6-position of thymine leading to the formation of thymine radical via radical cation or hydrolysis. This further reacts with PDP to give the final product, 5,6-dihydroxythymine.