SINGLE‐ and DOUBLE‐STRAND BREAK FORMATION IN DOUBLE‐STRANDED DNA UPON NANOSECOND LASER‐INDUCED PHOTOIONIZATION

Abstract— Double‐stranded (ds) calf thymus DNA (0.4 mM), excited by 20 ns laser pulses at 248 nm, was studied in deoxygenated aqueous solution at room temperature and pH 6.7 in the presence of a sodium salt (10 mM). The quantum yields for the formation of hydrated electrons (Φe.), single‐strand breaks (Φssb) and double‐strand breaks (Φdsb) were determined for various laser pulse intensities (I1). Φc. and Φssb increase linearly with increasing IL; however, Φssb has a tendency to reach saturation at high IL (Φ5 times 100 Wcm−2). The ratio Phi;ssh/Φc‐. representing the number of ssb per radical cation, is about 0.08 at IL 5 times 106 Wcm−2. For comparison, the number of ssb per OH radical reacting with dsDNA is 0.22. On going from argon to N2O saturation, Φssb and Φdsb become larger by factors of 5 and10–15, respectively. This enhancement is produced by attack on DNA bases by OH radicals generated by N2O‐scavenging of the photoelectrons. While Φssb is essentially independent of the dose (Etot), Φdsb, depends linearly on Etot in both argon‐ and N2O‐saturated solutions. The linear dependence of Φdsb implies a square dependence of the number of dsb on Etot. This portion of dsb formation is explained by the occurrence of two random ssb, generated within a critical distance (h) in opposite strands. For both argon‐ and N2O‐saturated solutions h was found to be of the order of40–70 phosphoric acid diester bonds. On addition of electron scavengers such as 2‐chloroethanoI (or N2O plus t‐butanol), Φdsb is similar to that in neat, argon‐saturated solutions. Thus, hydrated electrons are not involved in the chemical pathway leading to laser‐pulse‐induced dsb of DNA.