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

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 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●).

Radiation-Induced Oxidation Reactions of 2-Selenouracil in Aqueous Solutions: Comparison with Sulfur Analog of Uracil

“…Taking into account, reported in our earlier work, the effect of 2-thiouracil (2-TU) concentration on the transient absorption spectra [ 46 ], we decided to record UV-vis transient absorption spectra at various 2-SeU concentrations to check whether and to what extent concentration of 2-SeU affects spectral characteristics. The spectral changes observed after pulse irradiation of N 2 O-saturated unbuffered aqueous solutions at two pHs (4 and 10) containing 2-SeU in the concentration range of 0.05 mM to 1 mM are shown in Figure 1 .…”

“…Based on our earlier work on 2-TU [ 46 ], the transient absorption spectrum with λ max = 440 nm was tentatively assigned to the formation of dimeric radical with 2c-3e selenium-selenium bond. In principle, by analogy to 2-TU, the formation of the ● OH adducts at C5 and C6 positions leading to the respective C-centered radicals can also be expected.…”

“…This increase cannot be accounted for by an increase in ● OH concentration scavenged by 2-SeU due to the higher concentration of the latter [ 47 ]. Based on our earlier work with 2-TU [ 46 ], it rather points to the existence of an equilibrium, analogous to that in solutions containing 2-TU, where dimeric selenium radicals or radical anions are formed, which are responsible for the strong absorption at λ max = 440 nm.…”

Spectral Probe for Electron Transfer and Addition Reactions of Azide Radicals with Substituted Quinoxalin-2-Ones in Aqueous Solutions

“…The azide radical (N 3 ● ) is one of the most important one-electron oxidants used extensively in radiation chemistry studies involving inorganic [ 1 , 2 , 3 ], and aromatic compounds [ 4 , 5 , 6 , 7 , 8 , 9 ], and also molecules of biological significance [ 10 , 11 , 12 , 13 , 14 , 15 ]. In general, it is assumed that N 3 ● reacts in aqueous solution only by electron transfer [ 4 , 16 , 17 ].…”

“…Other interesting examples of using N 3 ● as one-electron oxidant were the studies involving sulfur-substituted nucleobases (thiobases). It was shown that the reactions of 4-thiouracil (4TU(=S)) [ 32 ], and 2-thiouracil (2TU(=S)) [ 15 ] with N 3 ● led to the respective dimeric 2c-3e S-S-bonded radicals in their cationic and neutral forms, respectively (Equations (4) and (5)). N 3 ● + 4TU(=S) → N 3 − + 4TU + (S ● ) 4TU + (S ● ) + 4TU(=S) ⇆ (4TUS∴S4TU) + N 3 ● + 2TU(=S) → N 3 − + 2TU + (S ● ) 2TU + (S ● ) → 2TUS ● + H + 2TU + (S ● ) + 2TU(=S) ⇆ (2TUS∴S2TU) …”