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) 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 = 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 = 1.3 × 10 M at pH 4 (below the pK of 2-SeU). Similar equilibrium with K = 4.4 × 10 M was determined for pH 10 (above the pK 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 = 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-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.