Quantum yields for the generation of hydrated electrons and single-strand breaks in poly(C), poly(A) and single-stranded DNA in aqueous solution on 20 ns laser excitation at 248 nm

Abstract

The quantum yields of formation of hydrated electrons (Φ e−) and single-strand breaks (Φ ssb) were determined for poly(C), poly(A) and single-stranded DNA (ssDNA) in deoxygenated aqueous solution at room temperature and pH values around 7 on 248 nm nanosecond laser excitation. In the intensity range I L = (0.5 − 3) × 10 6 W cm −2 and for nucleic acid concentrations of 0.4 mM, Φ ssb was found to increase almost linearly with I L and approaches a saturation value at I L > 5 × 10 6 W cm −2. The dependence of Φ e− on I L is also linear under comparable conditions. These results imply a quadratic dependence of both the number of single-strand breaks (ssb) and photoelectrons on I L. For I L = (1 − 5) × 10 6 W cm −2 the ratio Φ ssb:Φ e− is 0.4, 0.06 and 0.05 for poly(C), poly(A) and ssDNA, respectively. These values are interpreted as efficiencies of ssb formation per radical cation of the bases. Similar efficiencies have been reported for OH radical-induced ssb formation. It is concluded that, prior to the ssb formation, the radical cations are transformed into neutral successor radicals which are often the same as those produced by reaction of the OH radicals with the base moieties. For poly(C) and poly(A) in N 2O-saturated solutions at low intensities, Φ ssb is about twice as large as in argon-saturated solutions, indicating that the laser-induced OH radicals react randomly with the macromolecules. Hydrated photoelectrons are not involved in the chemical pathway leading to strand breakage since Φ ssb is not changed on addition to 2-chloroethanol. There is no indication that OH radicals, other than those generated by reaction of photoelectrons with N 2O, contribute to ssb formation. From the results it can be concluded that the so-called direct effect of DNA damage by absorption of high-energy radiation can lead chemically to the same damage as the indirect effect via OH radicals.