Laser-induced photoionization and single-strand break formation for polynucleotides and single-stranded DNA in aqueous solution: model studies for the direct effect of high energy radiation on DNA

Quantum yields for the formation of hydrated electrons (Φ_e−) and single-strand breaks (Φ_ssb) were measured for polyadenylic acid (poly A), polycytidylic acid (poly C), polyuridylic acid (poly U) and single-stranded (ss) DNA in deoxygenated aqueous solution at room temperature upon nanosecond laser excitation at 248 nm. The values Φ_e⁻ = (1.7 − 3) × 10⁻² and Φ_ssb = (0.5 − 5) × 10⁻⁴ at intensities of (0.5 − 1.5) × 10⁷ W cm⁻² were obtained. The results show that base radical cations of these polynucleotides and of ssDNA lead to ssb formation. The large difference between Φ_e⁻ and Φ_ssb (a factor of 60 – 380) is explained as being the result of back reactions of geminate pairs of radicals and/or radical ions. Conductivity changes after the laser pulse, representing ssb formation, reveal a fast increase and a consecutive show increase for all nucleic acids studied. The activation energies and pre-exponential factors, calculated from the rate constant k_obs of the slow conductivity increase at pH 7.3, are reported. Dithiothreitol (DTT) inhibits ssb formation of poly C (rate constant k_DTT = 1.7 × 10⁶ M⁻¹ s⁻¹) and poly U whereas for poly A and ssDNA a smaller effect was found (k_DTT < 0.7 × 10⁶ M⁻¹ s⁻¹. The pH dependence of k_obs is very similar for poly A and ssDNA, but different from that of poly C and poly U. This indicates that the slow strand break formation in ssDNA upon laser excitation is related to the purine moiety.