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

Oxidative damage to 2-thiouracil (2-TU) by hydroxyl ((OH)-O-center dot) and azide (N-center dot(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)-O-center dot 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 lambda = 325, 340 and 385 nm, whereas for high concentrations, they are dominated by an absorption band with lambda(max) approximate to 425 nm. Based on calculations using TD-DFT method, the transient absorption spectra at low concentration of 2-TU were assigned to the (OH)-O-center dot-adducts to the double bond at C5 and C6 carbon atoms (3(center dot), 4(center dot)) and 2c-3e bonded (OH)-O-center dot adduct to sulfur atom (1 ... (OH)-O-center dot) and at high concentration of 2-TU also to the dimeric 2c-3e S-S-bonded radical in neutral form (2(center dot)). The dimeric radical (2(center dot)) is formed in the reaction of thiyl-type radical (6(center dot)) with 2-TU and both radicals are in an equilibrium with K-eq = 4.2 x 10(3) M-1. Similar equilibrium (with K-eq = 4.3 x 10(3) M-1) was found for pH above the pK(a) of 2-TU which involves admittedly the same radical (6(center dot)) but with the dimeric 2c-3e S-S bonded radical in anionic form (2(center dot-)). In turn, N-center dot(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(center dot)). This radical is a direct precursor of dimeric radical (2(center dot)).