Journal ArticleDOI

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

23 Jul 2009-Journal of Physical Chemistry B (American Chemical Society)-Vol. 113, Iss: 29, pp 10008-10013

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.

AbstractIn 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.

Topics: Secondary electrons (61%)

## Summary (2 min read)

Jump to: [III. Results] and [V. Conclusion]

### 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.

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251 citations

Journal ArticleDOI

Abstract: Most of the low-energy electrons emitted from a material when it is subjected to ionization radiation are believed to be directly ionized secondary electrons. Coincidence measurements of the electrons ejected from water clusters suggests many are produced by a quantitatively new mechanism, known as intermolecular Coulombic decay. Low-energy electrons are the most abundant product of ionizing radiation in condensed matter. The origin of these electrons is most commonly understood to be secondary electrons1 ionized from core or valence levels by incident radiation and slowed by multiple inelastic scattering events. Here, we investigate the production of low-energy electrons in amorphous medium-sized water clusters, which simulate water molecules in an aqueous environment. We identify a hitherto unrecognized extra source of low-energy electrons produced by a non-local autoionization process called intermolecular coulombic decay2 (ICD). The unequivocal signature of this process is observed in coincidence measurements of low-energy electrons and photoelectrons generated from inner-valence states with vacuum-ultraviolet light. As ICD is expected to take place universally in weakly bound aggregates containing light atoms between carbon and neon in the periodic table2,3, these results could have implications for our understanding of ionization damage in living tissues.

213 citations

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Abstract: Autoionization is an important pathway for the relaxation of electronically excited states. In weakly bonded matter, efficient autoionization channels have been found, in which not only the initially excited state, but also neighbouring atoms or molecules take part. Since their theoretical prediction in 1997 these processes are known as Interatomic or Intermolecular Coulombic Decay (ICD). The author summarizes the experimental research on ICD up to the presence. Experiments on inner valence ICD in rare gas clusters, on cascade ICD after Auger decay and on ICD of satellite states are explicitly discussed. First experiments on water clusters and on solutes will be reviewed. An outlook on other non-local autoionization processes and on future directions of ICD research closes the article.

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TL;DR: The results indicate that DNA damage induced by LEEs does not increase significantly until the second hydration shell is formed, however, this damage increases dramatically as DNA coverage approaches bulk-like hydration conditions.
Abstract: We report the effect of the DNA hydration level on damage yields induced by soft X-rays and photoemitted low-energy electrons (LEEs) in thin films of plasmid DNA irradiated in N2 at atmospheric pressure under different humidity levels. Contrary to a dilute solution of DNA, the number of H2O molecules per nucleotide (Γ) in these films can be varied from Γ = 2.5 to ∼33, where Γ ≤ 20 corresponds to layers of hydration and Γ = 33 to an additional bulk-like water layer. Our results indicate that DNA damage induced by LEEs does not increase significantly until the second hydration shell is formed. However, this damage increases dramatically as DNA coverage approaches bulk-like hydration conditions. A number of phenomena are invoked to account for these behaviors, including dissociative electron transfer from water–interface electron traps to DNA bases, quenching of dissociative electron attachment to DNA, and quenching of dissociative electronically excited states of H2O in contact with DNA.

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03 Mar 2000-Science
TL;DR: It is shown that reactions of such electrons, even at energies well below ionization thresholds, induce substantial yields of single- and double-strand breaks in DNA, which are caused by rapid decays of transient molecular resonances localized on the DNA's basic components.
Abstract: Most of the energy deposited in cells by ionizing radiation is channeled into the production of abundant free secondary electrons with ballistic energies between 1 and 20 electron volts. Here it is shown that reactions of such electrons, even at energies well below ionization thresholds, induce substantial yields of single- and double-strand breaks in DNA, which are caused by rapid decays of transient molecular resonances localized on the DNA's basic components. This finding presents a fundamental challenge to the traditional notion that genotoxic damage by secondary electrons can only occur at energies above the onset of ionization, or upon solvation when they become a slowly reacting chemical species.

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TL;DR: By comparing the results from different experiments and theory, it is possible to determine fundamental mechanisms that are involved in the dissociation of the biomolecules and the production of single- and double-strand breaks in DNA.
Abstract: The damage induced by the impact of low energy electrons (LEE) on biomolecules is reviewed from a radiobiological perspective with emphasis on transient anion formation. The major type of experiments, which measure the yields of fragments produced as a function of incident electron energy (0.1-30 eV), are briefly described. Theoretical advances are also summarized. Several examples are presented from the results of recent experiments performed in the gas-phase and on biomolecular films bombarded with LEE under ultra-high vacuum conditions. These include the results obtained from DNA films and those obtained from the fragmentation of elementary components of the DNA molecule (i.e., the bases, sugar and phosphate group analogs and oligonucleotides) and of proteins (e.g. amino acids). By comparing the results from different experiments and theory, it is possible to determine fundamental mechanisms that are involved in the dissociation of the biomolecules and the production of single- and double-strand breaks in DNA. Below 15 eV, electron resonances (i.e., the formation of transient anions) play a dominant role in the fragmentation of all biomolecules investigated. These transient anions fragment molecules by decaying into dissociative electronically excited states or by dissociating into a stable anion and a neutral radical. These fragments can initiate further reactions within large biomolecules or with nearby molecules and thus cause more complex chemical damage. Dissociation of a transient anion within DNA may occur by direct electron attachment at the location of dissociation or by electron transfer from another subunit. Damage to DNA is dependent on the molecular environment, topology, type of counter ion, sequence context and chemical modifications.

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William W. Wilfinger
TL;DR: RNA A260/280 ratios are found to be more reliable and reproducible when these spectrophotometric measurements were performed at pH 8.0-8.5 and the ability to detect protein contamination was significantly improved when RNA wasSpectrophotometrically analyzed in an alkaline solution.
Abstract: The ratio of absorbance at 260 and 280 nm (the A260/280 ratio) is frequently used to assess the purity of RNA and DNA preparations. Data presented in this report demonstrate significant variability in the RNA A260/280 ratio when different sources of water were used to perform the spectrophotometric determinations. Adjusting the pH of water used for spectrophotometric analysis from approximately 5.4 to a slightly alkaline pH of 7.5-8.5 significantly increased RNA A260/280 ratios from approximately 1.5 to 2.0. Our studies revealed that changes in both the pH and ionic strength of the spectrophotometric solution influenced the A260/280 ratios. In addition, the ability to detect protein contamination was significantly improved when RNA was spectrophotometrically analyzed in an alkaline solution. UV spectral scans showed that the 260-nm RNA absorbance maximum observed in water was shifted by 2 nm to a lower wavelength when determinations were carried out in Na2HPO4 buffer at a pH of 8.5. We found RNA A260/280 ratios to be more reliable and reproducible when these spectrophotometric measurements were performed at pH 8.0-8.5 in 1-3 mM Na2HPO4 buffer.

412 citations

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TL;DR: Collisions of 0-4 eV electrons with thin DNA films are shown to produce single strand breaks, which support aspects of a theoretical study by Barrios et al. indicating that such a mechanism could produce strand breaks in DNA.
Abstract: Collisions of 0--4 eV electrons with thin DNA films are shown to produce single strand breaks. The yield is sharply structured as a function of electron energy and indicates the involvement of ${\ensuremath{\pi}}^{*}$ shape resonances in the bond breaking process. The cross sections are comparable in magnitude to those observed in other compounds in the gas phase in which ${\ensuremath{\pi}}^{*}$ electrons are transferred through the molecule to break a remote bond. The results therefore support aspects of a theoretical study by Barrios et al. [J. Phys. B 106, 7991 (2002)] indicating that such a mechanism could produce strand breaks in DNA.

393 citations

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TL;DR: The biochemistry of the nucleic acids is studied in detail in order to establish a clear picture of the role of phosphorous and nitrogen in the structure of DNA.
Abstract: The biochemistry of the nucleic acids , The biochemistry of the nucleic acids , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

388 citations

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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.