DNA Damage Induced By UV Photoelectrons

In the radiotherapy of cancer the cell damages are essentially caused by the action of vast secondary electrons and radiacals generated along the track of the high energy radiation. The low energy electrons (LEE), with generally energies below 20 eV and a mean energy of 10 eV, consist the majority of secondary electrons. The interaction of LEE with biomolecules thus plays the vital role in radiation research particularly for the efficiency improvement of the cancer radiotherapy. DNA serves as genetic codes in cells and organisms, which makes it the most significant target in the molecular biochemistry research. Due to the very short thermalization distances, the current research of DNA damage by low energy electrons are greatly limited in high vacuum, and challenged for how to carry on under the conditions simulating cellular environment involving water and atmosphere.In this study, the preliminary research on DNA damage induced by LEE under atmospheric environment has been achieved. Plasmid DNA was chose as the target loaded on different substrates. DNA films were prepared by lyophilization and transferred into a homemade setup where the DNA was illuminated by Xenon or Hg lamp. The photoelectrons can be excited by UV on the surface of Ta substrates, while the insulator quartz serves as the photon absorption control substrate. The photocurrents of different substrates are monitored by electrochemical workstation and the work functions for substrates with and without DNA were tested by Kelvin probe.The main results show:1) LEE can be induced by UV in atmospheric environment and lead to remarkable DNA damage.2) The factors, including as thermal effect, the flux of nitrogen and charging level of apparatus, can increase the experimental background. The stability of film surface is another important element to be considered.3) The photon flux is estimated to be (0.1-1.4)×1015 photons/(s·cm2) and the electron yield of (0.6-5.5)×10-4 electrons/photon(stable value of photocurrent), (0.2-2.1)×10-3 electrons/photon (peak value of photocurrent), respectively. The yield of DNA damage by electrons was 2.6×10-3 molecules/electron and the EF value is 1.4. 4) The G-values of UV light is obtained to be (0.2-1.1)×10-8 D/100 eV, while G-values of LEE is much higher of (0.20-0.72) D/100 eV.5) The possible mechanism of LEE to DNA was proposed that the interactions among DNA and very low energy photoelectrons (0-1 eV) form TNIs which decay (DEA process) to achieve the disruption of DNA C-O bonds by two possible resonance processes (Shape Resonance and Nuclear Feshbach Resonance).In summary, the present study for the first time attempts to address the answer to obtain the very low energy photoelectrons (0-1 eV) induced by UV and introduce the LEE to damage DNA in the atomospheric environment. The photoelectrons with energy below 1 eV exhibit high efficiency leading to the breakage of supercoiled DNA and the possible mechanism of DNA damage by LEE is also elucidated.