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

Oxidative damage to 2-thiouracil (2-TU) by hydroxyl (^•OH) and azide (^●N₃) 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 × 10³ M^-1. Similar equilibrium (with K_eq = 4.3 × 10³ 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, ^●N₃-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^●).