Uracil derivatives: sites and kinetics of protonation of the radical anions and the UV spectra of the C(5) and C(6) h-atom adducts

Solvated electrons from the radiolysis of water were reacted with a number of uracil derivatives. The radical anions I so formed undergo rapid protonation at O(4) (I + H+ → II) independent of the pyrimidine used. With the equilibrium I/II established a slower protonation occurs at C(6) (I + H2PO4- → III + HPO42-). Whereas the absorption maxima of I and II are less than or equal to 300 nm the absorption maxima of III for the various pyrimidines center around 400 nm. The rate constants of this C(6) protonation very much depend on the site of methyl substitution. Using tetranitromethane as a probe showed that the reducing radicals I and II are quantitatively converted by the slow protonation into the oxidizing radical III. These results clearly show the high selectivity of this protonation and exclude the possibility of any significant protonation on C(5). The relevance of these results for the radiation chemistry of DNA are discussed. 23 references, 8 figures, 2 tables