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

The rates of photooxidation of thymine in the presence of peroxydiphosphate (PDP) have been determined by measuring the absorbance of thymine at 264 n m spectrophotometrically. The rates and the quantum yields (phi) of oxidati on of thymine by phosphate radical anion have been determined in the presen ce of different concentrations of dithiothreitol (DTT). An increase in DTT is found to decrease the rate of oxidation of thymine, suggesting that DTT acts as an efficient scavenger of PO4.2- and protects thymine from it. Phos phate radical anion competes for thymine as well as DTT; the rate constant for the phosphate radical anion with DTT has been calculated to be 2.21 x 1 0(9) dm(3) mol(-1) s(-1), assuming the rate constant of phosphate radical a nion reaction with thymine as 9.6 x 10(7) dm(3) mol(-1) s(-1). The quantum yields of photooxidation of thymine have been calculated from the rates of oxidation of thymine and the light intensity absorbed by PDP at 254 nm, the wavelength at which PDP is activated to phosphate radical anion. From the results of experimentally determined quantum yields (phi (exptl)) and the q uantum yields calculated (phi (cl)), assuming DTT acts only as a scavenger of PO4.2- radicals. show that phi (exptl) values are lower than phi (cl) va lues. The phi' values, which are experimentally found quantum yield values at each DTT concentration and corrected for PO4.2- scavenging by DTT, are a lso found to be greater than phi (exptl) values. These observations suggest that the thymine radicals are repaired by DTT in addition to scavenging of phosphate radical anions. (C) 2001 John Wiley & Sons. Inc.