The photooxidation of thymine in the presence of peroxydiphosphate (PDP) in
aqueous solution at natural pH (-7.5) under anoxic conditions has been car
ried out in a quantum yield reactor using a high pressure mercury lamp. The
reactions have been followed spectrophotometrically by measuring the absor
bance 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 intensi
ty absorbed by PDP at 254nm. The intensity has been measured using the pero
xydisulphate (PDS) solution as a standard chemical actinometer. On the basi
s of experimental results, product analysis and existence of isosbestic poi
nt, a probable mechanism is suggested in which peroxydiphosphate anion on p
hotolysis gives phosphate radicals which initiate the reaction by adding to
C6-position of thymine leading to the formation of thymine radical via rad
ical cation or hydrolysis. This further reacts with PDP to give the final p
roduct, 5,6-dihydroxythymine.