Reaction of substituted pyrimidines with photochemically generated t-BuO* radicals.
Humans are exposed to various organic peroxides through chemical, pharmaceutical and cosmetic products. On photolysis, these peroxides produce alkoxyl radicals and hydroxyl radicals. The reaction of *OH radicals with DNA and its constituents have been extensively studied, but very little is known about the reactions of alkoxyl radicals with DNA and its constituents. In view of this, the oxidation of pyrimidine bases viz., thymine, uracil, cytosine, 5-bromouracil, 6-methyluracil and 1,3-dimethyluracil by t-BuO* radicals in aqueous solution at pH 7.5 has been carried out. The reaction between pyrimidine and t-BuO* is followed by measuring the absorbance of pyrimidine at the respective lambdamax. The rates of oxidation of pyrimidines are calculated from the plot of absorbance vs time. The rates of oxidation of pyrimidines have been found to increase with increase in [t-BuOOH], [pyrimidine] and light intensity. The quantum yields are calculated from the initial rates of oxidation of pyrimidine and the measured light intensity at 254 nm the wavelength at which t-BuOOH is activated to give radicals. The quantum yields are found to depend on [pyrimidine] as well as on [t-BuOOH] while they are independent of light intensity. The product analysis was carried out on HPLC with UV-visible detector. The corresponding 5,6-dihydroxypyrimidine and isobarbituric acid have been identified by comparing the retention times of the authentic samples. On the basis of experimental results and product analysis, it is suggested that t-BuOOH on photolysis gives t-BuO* radical, which initiates the reaction by adding to C (5) or C (6) position of pyrimidine base, leading to the formation of pyrimidine base radical via hydrolysis. The pyrimidine radical further reacts with t-BuO* radical to give the final product. This study predicts the probable transient pyrimidine radicals.