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

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Abstract
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 Φssbincrease linearly with increasing IL; however, Φssbhas a tendency to reach saturation at high IL(Φ5 times 100Wcm−2). The ratio Phi;ssh/Φc‐. representing the number of ssb per radical cation, is about 0.08 at IL5 times 106Wcm−2. For comparison, the number of ssb per OH radical reacting with dsDNA is 0.22. On going from argon to N2O saturation, Φssband Φdsbbecome 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 Φssbis essentially independent of the dose (Etot), Φdsb, depends linearly on Etotin both argon‐ and N2O‐saturated solutions. The linear dependence of Φdsbimplies 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 solutionshwas 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), Φdsbis 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.