Development of a simulation method for dynamics of electrons ejected from DNA molecules irradiated with X-rays

Purpose: To develop a method for simulating the dynamics of the photoelectrons and Auger electrons ejected from DNA molecules irradiated with pulsed monochromatic X-rays. Materials and methods: A 30-base-pair (bp) DNA molecule was used as the target model, and the X-rays were assumed to have a Gaussian-shaped time distribution. Photoionization and Auger decay were considered as the atomic processes. The atoms from which the photoelectrons or Auger electrons were emitted were specified in the DNA molecule (or DNA ion) using the Monte Carlo method, and the trajectory of each electron in the electric field formed around the positively charged DNA molecule was calculated with a Newtonian equation. The kinetics of the electrons produced by irradiation with X-rays at an intensity ranging from 1 × 1012 to 1 × 1016 photons/mm² and energies of 380 eV (below the carbon K-edge), 435 eV (above the nitrogen K-edge), and 560 eV (above the oxygen K-edge) were evaluated.Results: It was found that at an X-ray intensity of 1 × 1014 photons/mm² or less, all the produced electrons escaped from the target. However, above an X-ray intensity of 1 × 1015 photons/mm² and an energy of 560 eV, some photoelectrons that were ejected from the oxygen atoms were trapped near the target DNA.Conclusions: A simulation method for studying the trajectories of electrons ejected from a 30-bp DNA molecule irradiated with pulsed monochromatic X-rays has been developed. The present results show that electron dynamics are strongly dependent on the charged density induced in DNA by pulsed X-ray irradiation. (author)