The Reactions of the 2′-deoxyadenosine Electron Adduct in Aqueous Solution. The Effects of the Radiosensitizer p-nitroacetophenone. A Pulse Spectroscopic and Pulse Conductometric Study

Summary

The reactions of the one electron adduct of 2′-deoxyadenosine dA, in aqueous solutions have been studied using pulse radiolysis techniques with optical absorption and conductivity detection. The dA radical anion itself shows a weak and featureless optical absorption at > 300 nm. It reacts rapidly with H₂O (k = 6 × 10⁴ M⁻¹ s⁻¹; t_1/2 = 210 ns) to yield at least three different protonated structures (dAH^·, dA′H^·, dA″H^·.). In neutral solutions the most important of these (dAH^·, λ(max) = 315 nm) decays by a first order process (t_1/2 ≈ 9 μs). In basic solutions dAH^· undergoes an OH⁻ catalysed rearrangement into another neutral radical (dA′″ H^·, λ(max) = 355 nm; k(dAH^· + OH⁻) = 1·7 × 10⁸ M⁻¹ s⁻¹).

p-Nitroacetophenone (PNAP) reacts rapidly with the protonated electron adducts of 2′-deoxyadenosine (k = 5 × 10⁹ M⁻¹ s⁻¹). An electron transfer occurs with dAH^· to yield PNAP and a reoxidized form of 2′-deoxyadenosine. As indicated by its pK value of 8·8 the latter is not, however, simply a repaired 2′-deoxyadenosine molecule, but is suggested to include the elements of water. Species dA′″H^· (and dA′H^·) react with PNAP in a process which is clearly not an electron transfer but likely an addition reaction.