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.