Damage induced to DNA by low-energy (0-30 eV) electrons under vacuum and atmospheric conditions.

TL;DR: It is shown that it is possible to obtain data on DNA damage induced by low-energy (0-30 eV) electrons under atmospheric conditions and the differences in damage yields recorded with the gold and glass substrates is essentially attributed to the interaction of low- energy electrons with DNA under vacuum and hydrated conditions.

Abstract: In this study, we show that it is possible to obtain data on DNA damage induced by low-energy (0-30 eV) electrons under atmospheric conditions. Five monolayer films of plasmid DNA (3197 base pairs) deposited on glass and gold substrates are irradiated with 1.5 keV X-rays in ultrahigh vacuum and under atmospheric conditions. The total damage is analyzed by agarose gel electrophoresis. The damage produced on the glass substrate is attributed to energy absorption from X-rays, whereas that produced on the gold substrate arises from energy absorption from both the X-ray beam and secondary electrons emitted from the gold surface. By analysis of the energy of these secondary electrons, 96% are found to have energies below 30 eV with a distribution peaking at 1.4 eV. The differences in damage yields recorded with the gold and glass substrates is therefore essentially attributed to the interaction of low-energy electrons with DNA under vacuum and hydrated conditions. From these results, the G values for low-energy electrons are determined to be four and six strand breaks per 100 eV, respectively.

Summary

Atmospheric Conditions

• Émilie Brun,† Pierre Cloutier,‡ Cécile Sicard-Roselli,† Michel Fromm,§ and Léon Sanche*,†,‡ Laboratoire de Chimie Physique, CNRS UMR 8000, UniVersité Paris-Sud 11, Bât.
• The authors present knowledge of LEE-biomolecule interactions arises from both theoretical and experimental investigations.
• Molecules that could be evaporated in a vacuum environment without decomposition have usually been studied as gases, but some studies have also been reported on solid molecular films.
• 23,24 The apparatus is equipped with an Al KR X-ray source, but the metal target can be replaced for X-ray emission at other characteristic energies.
• To delineate the portion of DNA damage caused by X-rays and that arising from LEE interactions, the authors performed experiments with films deposited on an insulator and the electron-emitting gold surface under different environmental conditions.

II. Experimental Methods

• PGEM-3Zf(-) plasmid DNA (3197 base pairs, Promega) was extracted from Escherichia coli DH5R and purified with the QIAfilter Plasmid Giga Kit .
• The stock solution concentration was approximatively 50 ng ·µL-1. DNA purity was checked by recording the ratio between absorbances at 260 and 280 nm.27-29 Sample Preparation.
• The lyophilized samples were exposed to the Al KR X-rays produced, under atmospheric conditions, from a cold-cathode transmission target X-ray tube.
• The discharge electron current is controlled by the stabilized circulation of the N2 gas with the leak valve.
• The absorbed dose rate in water, according to the ionization chamber measurement, was 2.1 Gy ·min-1. A linear relationship between log10(I0/I) and the dose was obtained in the range 0-100 Gy.

III. Results

• Within experimental error, the loss of the supercoiled form is a linear function of the photon fluence.
• The enhancement factors (EF) derived from these values appear on the right.
• As expected, the gold substrate enhances DNA damage.
• The percentage yields derived from the slope of these curves are given in the second line of Table 1.
• For 0-30 eV SE, the energy distribution η(Ek) was calculated using33 where ηs is a coefficient that normalizes the yield of SEs having kinetic energy of Ek and W is the work function of gold, that is, 4.8 eV.34 Ninety-six percent of these SEs have energies below 30 eV, and the average energy for these electrons is 5.9 eV.

IV. Discussion

• Given the mass absorption coefficient of DNA36 and the formula for transmitted photons (Xtrans in the Supporting Information), one can calculate that within a 5 ML film, 0.2% of 1.5 keV photons interact with DNA, while the rest pass through DNA.
• Thus, DNA damage is induced by both X-ray photons and LEEs when DNA lies on a gold substrate.
• In the presence of water, the G value of LEEs further increases by 50% whereas that of X-rays remains the same within instrumental error.
• In dilute solution of DNA, the hydroxyl radical (OH) is considered to be the secondary species formed by water radiolysis that produces the largest amount of DNA damage.
• The D(2S), O(3P2), and O(1D2) yields versus incident electron energy have an apparent threshold at ∼6.5 eV with a steadily increasing intensity.

V. Conclusion

• The authors have shown that photoelectrons emitted from a gold substrate can be used as a source of low-energy electrons (LEEs) to irradiate DNA films under atmospheric conditions.
• LEE damage to plasmid DNA with its hydratation shell was measured from comparison of results obtained with films deposited on gold and glass substrates.
• The authors thank Ariane Dumont for providing us with the plasmid.
• Details about of the calculations of the G values for photons and low-energy electrons, also known as Supporting Information Available.
• This material is available free of charge via the Internet at http://pubs.acs.org.

Figures

Figure 1. Schematic overview of the type of apparatus used to irradiate samples with 1.5 keV Al KR X-ray photons under atmospheric conditions: (A) baratron, (B) adjustable leak valve, (C) concave aluminum cathod, (D) high voltage electric feedthrough, (E) aluminum foil, (F) atmosphere, (G) glass-ceramic support, (H) quartz tube.

Figure 4. Energy spectrum of induced secondary electron emission from a gold surface.

TABLE 1: Percentage Yield per 1012 photons · cm-2 for the Loss of Supercoiled DNA in 5 ML Films Deposited on Glass and Gold Substrates and Held under Vacuum or Atmospheric Conditionsa

Figure 3. X-ray exposure curves for the loss of the supercoiled DNA under atmospheric conditions on glass and gold substrates. The points represent the means of three independent experiments and the error bars represent standard deviation of the means.

Figure 2. X-ray exposure curves for the loss of the supercoiled DNA in vacuum on glass and gold substrates. The points represent the means of three independent experiments and the error bars represent standard deviation of the means.

Frequently asked questions

Q1. What are the contributions in "Damage induced to dna by low-energy (0-30 ev) electrons under vacuum and atmospheric conditions" ?

Brun et al. this paper showed that photoelectrons emitted from a gold substrate can be used as a source of low-energy electrons ( LEEs ) to irradiate DNA films under atmospheric conditions.