A Pulse Radiolysis Study of the Free Radical Intermediates in the Radiolysis of Uracil

Abstract

The technique of pulse radiolysis has been utilized to study the absorption spectra of the transient free radical adducts which result from reactions of ·OH and $e_{{\rm aq}}{}^{-}$ with the pyrimidine, uracil. The ·OH attaches to the 5,6 bond of uracil to form an adduct (·UOH) which has peaks at 390 nm and ∼240 nm. Changes in the spectrum of ·UOH with pH have led us to the conclusion that at least one isomer of ·UOH has ${\rm pK}_{{\rm a}}\text{'}$s of ∼7 and 9.5. The ·UOH decays by a second order kinetics with rate constant, $2k=(1.1\pm 0.2)\times 10^{9} M^{-1} s^{-1}$ which is independent of ionic strength. There is a residual spectrum which remains after the decay of the ·UOH absorption. This residual spectrum is different at pH 5 and 8.5, and shows a similar pH dependence to that of ·UOH itself. The $e_{{\rm aq}}{}^{-}$ adds to uracil to form an adduct which has protonated ( $\cdot {\rm UH}_{1}$) and unprotonated ( $\cdot {\rm U}^{-}$) forms which are in equilibrium with a pK a of 7. The spectra of...