Spin-trapping studies of the reaction of the sulfate radical anion with N1-substituted pyrimidine bases. Comparison with continuous-flow electron paramagnetic resonance experiments

Abstract

Photolysis of K₂S₂O₈-containing solutions of N¹-substituted pyrimidines (1-methyluracil, 1-MU; 1, 3-dimethyluracil, l, 3-DMU;1-methylthymine, 1-MT;1, 3-dimethylthymine, 1,3-DMT;1-methylcytosine, 1-MC; 2′-deoxycytosine, dC), together with the spin trap 2-methyl-2-nitrosopropane (MNP) resulted in the EPR spectra of persistent nitroxyl radicals. They were due to spin adducts of 5-hydroxy-5, 6-dihydropyrimidin-6-yl radicals for 1-MU and 1, 3-DMU and to 4-hydroxy-5, 6-dihydropyrimidin-5-yl radicals for 1-MT and 1, 3-DMT. Spectra obtained from 1-MC and dC were tentatively assigned to ring-opened products. Spin-trapped phosphate adduct radicals generated in the presence of HPO₄^2– were characterized by β-nitrogen and β-proton hyperfine splittings and/or by signal intensities. These results are consistent with one-electron oxidation of the substrates by SO₄˙^– and rapid reaction of the ensuing base radical cations with water or phosphate. Secondary reactions such as oxidation of 5-hydroxy-5, 6-dihydrouracil-6-yl radicals by S₂O₈^2– or rearrangement of 5-phosphate-5, 6-dihydrouracil-6-yl radicals complicating the interpretation of previously reported continuous flow EPR spectra (see ref. 22), were not observed in the spin-trapping experiments.