Ultraviolet and γ-Ray-Induced Free-Radical Reactions of Nucleic acid Constituents. Selectivity of Some Reactions for Purines. Suppression of the Reactivity of Pyrimidines

The photochemical reactions of purine and pyrimidine bases, nucleosides, nucleotides, and polynucleotides with 2-propanol, employing di-tert-butyl peroxide as initiator, were found to be selective for purines. The selectivity was shown to result from the suppression of the reactivity of the pyrimidines due to the presence of the purines. This was evidenced by comparison of the quantum yields for product formation, when the bases were allowed to react separately, to those in mixtures. In equimolar mixtures of pyrimidines and purines, the pyrimidine reactivity was suppressed, whereas that of the purine remained unchanged. The concentration and temperature dependence and the effect of anions on the degree of suppression of pyrimidine's reactivity in the presence of purines, as well as the correlation with changes in the osmotic coefficients upon dilution of mixtures, suggest that heteroassociation in the form of base-stacking is responsible for the effect. The thermally initiated reactions exhibit the same effect, thus indicating the general scope of the phenomenon.