Kinetics and Mechanism of Protection of Thymine from Phosphate Radical Anion under Anoxic Conditions

摘要

The rates of photOOxidation of thymine in the presence of peroxydiphosphate (POP) have been determined by measuring the absorbance of thymine at 264 nm spectropho- tometrically. The rates and the quantum yields (#psi#) of oxidation of thymine by phosphate radical anion have been determined in the presence of different concentrations of dithiothre- itol (DTT). An increase in DTT is found to decrease the rate of Oxidation of thymine. Suggesting that DTT acts as an efficient scavenger of PO_4~(~centre dot 2-) and protects thymine from it. Phosphate radical anion competes for thymine as well as on; the rate constant. fpr the phosphate radical anion with on has been calculated to be 2.21 * 10~9 dm~3 mol~(-l) S~(-1), assuming the rate constant of phosphate radical anion reaction with thymine as 9.6 * 10~7 dm~3 mol~(-1) S~(-1). The quantum yields of photo oxidation 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 resuJts of experimentally determined quantum yields ( #'psi#_exptl) and the quantum yields calculated (#psi#_cl), assuming DTT acts only as a scavenger of PO_4(~centre dot2 radicals, show that #psi#_exptl values are lower than #psi#_cl values. The #psi#_' values, which are experimen- tally found quantum yieJd values at each on concentration and corrected for PO_4~(centre dot2-) scavenging by DTT, are aJso found to be greater than #psi#_exptal values. These observations suggest that the thymine radicals are repaired by DTT in addition to sCavenging of phosphate radical anions.