Radiation-Induced Oxidation Reactions of 2-Selenouracil in Aqueous Solutions: Comparison with Sulfur Analog of Uracil

One-electron oxidation of 2-selenouracil (2-SeU) by hydroxyl ((●)OH) and azide ((●)N(3)) radicals leads to various primary reactive intermediates. Their optical absorption spectra and kinetic characteristics were studied by pulse radiolysis with UV-vis spectrophotometric and conductivity detection and by the density functional theory (DFT) method. The transient absorption spectra recorded in the reactions of (●)OH with 2-SeU are dominated by an absorption band with an λ(max) = 440 nm, the intensity of which depends on the concentration of 2-SeU and pH. Based on the combination of conductometric and DFT studies, the transient absorption band observed both at low and high concentrations of 2-SeU was assigned to the dimeric 2c-3e Se-Se-bonded radical in neutral form (2(●)). The dimeric radical (2(●)) is formed in the reaction of a selenyl-type radical (6(●)) with 2-SeU, and both radicals are in equilibrium with K(eq) = 1.3 × 10(4) M(−1) at pH 4 (below the pK(a) of 2-SeU). Similar equilibrium with K(eq) = 4.4 × 10(3) M(−1) was determined for pH 10 (above the pK(a) of 2-SeU), which admittedly involves the same radical (6(●)) but with a dimeric 2c-3e Se-Se bonded radical in anionic form (2(●−)). In turn, at the lowest concentration of 2-SeU (0.05 mM) and pH 10, the transient absorption spectrum is dominated by an absorption band with an λ(max) = 390 nm, which was assigned to the (●)OH adduct to the double bond at C5 carbon atom (3(●)) based on DFT calculations. Similar spectral and kinetic features were also observed during the (●)N(3)-induced oxidation of 2-SeU. In principle, our results mostly revealed similarities in one-electron oxidation pathways of 2-SeU and 2-thiouracil (2-TU). The major difference concerns the stability of dimeric radicals with a 2c-3e chalcogen-chalcogen bond in favor of 2-SeU.