DIRECT EVIDENCE FOR THE FORMATION OF THYMINE RADICAL CATIONS FROM THE REACTION OF SO4- WITH THYMINE DERIVATIVES - A PULSE-RADIOLYSIS STUDY WITH OPTICAL AND CONDUCTANCE DETECTION

A number of N(1)-substituted thymine derivatives have been oxidized in aqueous solution using the one-electron oxidant SO4•-. The rate constants for this reaction are in the range 1 × 109 mol-1 dm3 s-1 (5′-thymidylic acid) to 5 × 109 mol-1 dm3 s-1 (1-methylthymine). It is proposed that in all cases a radical cation is the initial transient observed. The 1,3-dimethylthymine radical cation decays by a first-order process (tt/2 ∼ 2 μs) with the production of a proton and can be followed by both absorbance and conductance measurements. This process is made up of water addition at C(6) and deprotonation from the C(5) methyl group. In alkaline solution the rate constant for the decay of the radical cation is increased, with OH- addition at C(6) being preferred to deprotonation from the C(5) methyl group. When N(3) is unsubstituted, deprotonation of the radical cation can occur to give the N(3)-centered radical. The pKa values for thymidine and 1-methylthymine radical cations are 3.6 and 3.8, respectively, i.e., more than 5 orders of magnitude lower than those of their parent compounds. These radical cations also decay by a first-order process, again involving both water addition at C(6) and deprotonation from the C(5)-methyl group (t1/2 = 0.9 and 2.4 μs, respectively, for the radical cations of thymidine and 1-methylthymine). Around neutral pH essentially only the N(3)-centered radicals are present and these decay bimolecularly (2k ≈ 109 mol-1 dm3 s-1). The first-order decay of the radical cations is unaffected by the presence of oxygen (1.3 × 10-3 mol dm-3). On the other hand the carbon-centered radicals produced in this decay react rapidly (close to diffusion controlled) with oxygen. The N(3)-centered radical reacts with oxygen with a rate constant of only 3.6 × 107 mol-1 dm3 s-1. © 1990 American Chemical Society.