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Open AccessFeature PaperArticle

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

1
Centre of Radiation Research and Technology, Institute of Nuclear Chemistry and Technology, 03-195 Warsaw, Poland
2
Faculty of Chemistry, Adam Mickiewicz University, 61-614 Poznan, Poland
3
Notre Dame Radiation Laboratory, University of Notre Dame, Notre Dame, IN 46556, USA
*
Authors to whom correspondence should be addressed.
Academic Editor: Chryssostomos Chatgilialoglu
Molecules 2019, 24(23), 4402; https://doi.org/10.3390/molecules24234402
Received: 6 October 2019 / Revised: 27 November 2019 / Accepted: 27 November 2019 / Published: 2 December 2019
(This article belongs to the Special Issue Biomimetic Radical Chemistry and Applications)
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 λmax ≈ 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 × 103 M−1. Similar equilibrium (with Keq = 4.3 × 103 M−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). View Full-Text
Keywords: 2-thiouracil; radiosensitizers; OH and N3 radicals; 2c-3e S∴S-bonded intermediates; pulse radiolysis; TD-DFT methods; thiobases; nucleobase derivatives 2-thiouracil; radiosensitizers; OH and N3 radicals; 2c-3e S∴S-bonded intermediates; pulse radiolysis; TD-DFT methods; thiobases; nucleobase derivatives
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MDPI and ACS Style

Skotnicki, K.; Taras-Goslinska, K.; Janik, I.; Bobrowski, K. Radiation Induced One-Electron Oxidation of 2-Thiouracil in Aqueous Solutions. Molecules 2019, 24, 4402.

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