OH Radical Reaction of 5-Substituted Uracils:  Pulse Radiolysis and Product Studies of a Common Redox-Ambivalent Radical Produced by Elimination of the 5-Substituents

Using a redox titration method in the pulse radiolysis of N2O-saturated phosphate buffer solution, the redox properties of the intermediate radicals derived from OH radical reactions of 5-substituted uracils 1c − h (R = F, Cl, Br, NO2, NH2, OH) have been characterized. While the primary intermediates were oxidizing C(5) radicals 2c − h and reducing C(6) radicals 3c − h of the respective OH adducts, a common allyl-type radical of “redox ambivalence” with limiting mesomeric forms of reducing carbon-centered 5-oxo-5,6-dihydrouracil-6-yl radical (4) and oxidizing oxygen-centered uracil-5-oxyl radical (5) was secondarily formed via elimination of 5-substituents from 3c − h. The rate of 5-substituent elimination decreased with varying structures of the 5-substituents in the order halo > amino, nitro > hydroxy groups. The common radical 4/5 showed weaker reducing/oxidizing abilities than the typical pyrimidine C(6) and C(5) radicals. The product study demonstrated that the one-electron reduction of 5-oxyl radical 5 by N,N,N‘,N‘-tetramethyl-p-phenylenediamine ((2.0 ± 0.1) × 108 dm3 mol-1 s-1) increased the yield of isobarbituric acid (1h), while the one-electron oxidation of 5-oxo C(6) radical 4 by tetranitromethane increased the yield of isodialuric acid (6).