Electron Transfer from Nucleobase Electron Adducts to 5-bromouracil. Is Guanine an Ultimate Sink for the Electron in Irradiated DNA?

Abstract

Electron transfer to 5-bromouracil (5-BrU) from nucleobase (N) electron adducts (and their protonated forms) has been studied by product analysis and pulse radiolysis. When an electron is transferred to 5-BrU, the ensuing 5-BrU radical anion rapidly loses a bromide ion; the uracilyl radical thus formed reacts with added t-butanol, yielding uracil. From the uracil yields measured as the function of [N]/[5-BrU] after γ-radiolysis of Ar-saturated solutions it is concluded that thymine and adenine electron adducts and their heteroatom-protonated forms transfer electrons quantitatively to 5-BrU. Like the electron adduct of adenine, those of cytosine and guanine are rapidly protonated by water. The (protonated) electron adduct of guanine does not transfer an electron to 5-BrU, and in the case of the (protonated) cytosine electron adduct only partial electron transfer is observed. The results can be modelled if the protonated electron adduct (protonated at N(3) or at the amino group) of cytosine, CH^·, which can transfer its electron to 5-BrU (k ≈ 2 × 10⁷ dm³ mol⁻¹ s⁻¹) is transformed in a slow tautomerization reaction (k ≈ 2·5 × ± 10³ s⁻¹) into another form C−H^· (possibly protonated at C(6) or C(5)) which does not transfer an electron to 5-BrU.