Pulse Radiolytic Studies of Radicals Produced from Methylated Uracils via Oxidation by SO₄⁻

Using a pulse radiolysis technique, some nucleic base radicals were produced by the reactions of sulfate radical, <tex-math>${\rm SO}_{4}^{-}$</tex-math>, with 1-, 3-, 5-, and 6-methyluracils, and their optical and kinetic natures were observed. All of their absorption spectra showed main peaks at ?400 nm with absorption constants ranging from 1020 to <tex-math>$\sim 1560\ {\rm dm}^{3}\ {\rm mol}^{-1}\ {\rm cm}^{-1}$</tex-math>. The rate constants of their formation were 1.6 to <tex-math>$\sim 3.3\times 10^{9}\ {\rm dm}^{3}\ {\rm mol}^{-1}\ {\rm s}^{-1}$</tex-math>. For thymine and 6-methyluracil, the absorption coefficients of their radicals at ?500 nm changed according to pH, giving pK values of ?9. For N(3)-methylated uracil, on the other hand, no such acid-base equilibrium was found. When the N(1) position was methylated, another type of pH effect was found. From these spectral observations and the comparative discussions, it was shown that methylation at the N(1) position gives OH-adduct radicals and at other positions proton-released radicals. For 3- and 6-methyluracils, second intermediates were formed concomitantly with the disappearance of the initial radicals. They are tentatively assigned to their ring-opened radicals, presumably by the reaction of the initial radicals with <tex-math>${\rm S}_{2}{\rm O}_{8}^{2-}$</tex-math>.