DNA strand breaks by direct energy deposition by Auger and photo-electrons ejected from DNA constituent atoms following K-shell photoabsorption.

PURPOSE: To study DNA strand breaks induced by direct energy deposition by photo- and Auger electrons ejected following K-shell photoabsorption of DNA constituent atoms (carbon, nitrogen, oxygen and phosphorus).

METHOD: Using a Monte Carlo code which has been developed to simulate the photoelectric effect on plasmid DNA pBR322, the energy deposition pattern of secondary electrons ejected after photoabsorption in DNA constituent atoms (not including the hydration shell) was calculated. Experimentally obtained X-ray absorption near edge structures were considered of the cross-sections at the K-shell resonant absorption (1 s-->sigma*) of carbon, nitrogen and oxygen, and the K-shell resonant absorption (1 s-->t2*) of phosphorus. Direct energy deposition by secondary electrons was scored using two different DNA models with and without the hydrated shell. The yields of SSB, DSB per photoabsorption events as well as break complexity were estimated for monochromatic X-rays around the K-edges of DNA constituent atoms (200-3000 eV).

RESULTS: Higher SSB and DSB yields were obtained below the carbon, nitrogen and oxygen K-edges compared to at or above the resonance, and at the K-shell resonant absorption of phosphorus compared to below the resonance. The number of electrons with sufficient energy to induce strand breaks was found to change depending on the photon energy. Electrons with 120 and 250 eV are shown to be rather more effective in SSB and DSB induction than electrons with higher energy. Inclusion of hydrated water in the DNA volume did not affect the photon energy dependence of the strand break yields.

CONCLUSION: The small difference of photon energies around K-absorption edges of the carbon, nitrogen, oxygen and phosphorus is indicated to induce variation in strand break yields by direct effect. Higher SSB and DSB induction efficiencies could be due to a higher yield of more than two electrons with around 120 eV to 250 eV per photoabsorption event.