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

Authors:

• Konrad Skotnicki,
• Katarzyna Małgorzata Taras-Goślińska,
• Ireneusz Janik,
• Krzysztof Bobrowski

Abstract

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 atλ=325, 340 and 385 nm, whereas for high concentrations, they are dominated by an absorption band withλmax ≈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 Keq =4.2×10 3M −1. Similar equilibrium (with Keq =4.3×10 3M −1) was found for pH above the pKa 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 •).