Soft X-ray and Low Energy Electron-Induced Damage to DNA under N₂ and O₂ Atmospheres

Abstract

DNA damage induced by low energy electrons (LEEs) and soft X-rays is measured under dry nitrogen and oxygen at atmospheric pressure and temperature. Five-monolayer plasmid DNA films deposited on tantalum and glass substrates are exposed to Al K α X-rays of 1.5 keV in the two different environments. From the damage yields for DNA, G-values are extracted for X-rays and LEEs. The G values for LEEs are 3.5 and 3.4 higher than those for X-ray photons under N 2 and O 2 atmospheres, respectively. Since most of the measured damage is in the form of single strand breaks (SSB), this result indicates a much higher effectiveness for LEEs relative to X-rays in causing SSB in both environments. The results indicate that the oxygen fixation mechanism, which is highly effective in increasing radiobiological effectiveness, under aerobic conditions, is operative on the type of damage created at the early stage of DNA radiolysis by LEEs.

Induction of strand breaks in DNA films by low energy electrons and soft X-ray under nitrous oxide atmosphere

“…Samples were frozen at −70°C and then freeze-dried by pumping under the pressure of 1-3 mTorr for two hours. The film thickness has been estimated as previously described (Alizadeh et al 2011;Goodhead, 1990) by taking 6.0 ± 0.2 mm as the diameter of the DNA film, a thickness of 2 nm for each ML of plasmid and the known density of 1.71 g/cm 3 of the plasmid extracted from E. coli (Adams et al, 1986). This procedure resulted in a dry film of 10 nm average thickness on the substrates, assuming minimal clustering of the plasmids.…”

“…All the experimental data were recorded with the apparatus recently developed by Alizadeh et al (2011) which has already been described in detail. Only a short survey is given here.…”

“…The number of incident photons on the samples or photon fluences, has been determined to be 1.14 × 10 10 photons/s.cm 2 using GAFCHROMIC ® HD-810 dosimetry films (Cai et al, 2003). Calibration of the sensitivity of Gafchromic film was obtained by irradiating film samples with X-rays produced by a PHI Model 04-548 Dual Anode X-ray photoelectron spectrometer (XPS, Perkin-Elmer) with an aluminum target and a factor 0.0393 min −1 , was used to convert the irradiation time to photon fluence (photon/cm 2 ) (Alizadeh et al, 2011).…”

“…Thus, to better understand the damaging effects of LEE on DNA, attempts have been made to perform LEE-irradiation experiments under conditions more closely approximating those found in the cell. For this reason, the effect of water and oxygen molecules on radiationinduced damage in biological systems has been extensively studied (Yokoya et al, 2002;Quintiliani, 1986;Yokoya et al 2009;Alizadeh et al 2011). Also in our laboratory some experiments have been completed under well-controlled environmental conditions at standard atmospheric temperature and pressure (STP) to show the induction of DNA damage by LEEs in presence of gases such as, air, pure N 2 and O 2 .…”

“…In preliminary experiments, Cai et al (2005Cai et al ( , 2006 studied the damage to DNA in vacuum resulting from the emission of secondary LEEs from a metal surface exposed to X-rays. Then, by improving the technique of Cai et al (2006), Brun et al (2009) and Alizadeh et al (2011) were able to investigate dry plasmid DNA films irradiated at STP by LEEs (0-30eV) and soft X-rays (1.5 keV) under air, N 2 and O 2 . Due to these advances in the technique it is now possible to study the influence of various gaseous atmospheres at STP on LEEs induced DNA damage.…”