Radiation Induced One-Electron Oxidation of 2-Thiouracil in Aqueous Solutions

Oxidative damage to 2-thiouracil (2-TU) by hydroxyl ((•)OH) and azide ((●)N(3)) radicals produces 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 time-dependent density functional theory (TD-DFT) method. The transient absorption spectra recorded in the reactions of (•)OH with 2-TU depend on the concentration of 2-TU, however, only slightly on pH. At low concentrations, they are characterized by a broad absorption band with a weakly pronounced maxima located at λ = 325, 340 and 385 nm, whereas for high concentrations, they are dominated by an absorption band with λ(max) ≈ 425 nm. Based on calculations using TD-DFT method, the transient absorption spectra at low concentration of 2-TU were assigned to the (●)OH-adducts to the double bond at C5 and C6 carbon atoms (3(●), 4(●)) and 2c-3e bonded (●)OH adduct to sulfur atom (1…(●)OH) and at high concentration of 2-TU also to the dimeric 2c-3e S-S-bonded radical in neutral form (2(●)). The dimeric radical (2(●)) is formed in the reaction of thiyl-type radical (6(●)) with 2-TU and both radicals are in an equilibrium with K(eq) = 4.2 × 10(3) M(−1). Similar equilibrium (with K(eq) = 4.3 × 10(3) M(−1)) was found for pH above the pK(a) of 2-TU which involves admittedly the same radical (6(●)) but with the dimeric 2c-3e S-S bonded radical in anionic form (2(●−)). In turn, (●)N(3)-induced oxidation of 2-TU occurs via radical cation with maximum spin location on the sulfur atom which subsequently undergoes deprotonation at N1 atom leading again to thiyl-type radical (6(●)). This radical is a direct precursor of dimeric radical (2(●)).