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ABSTRACT: It is well known that electrons below 15 eV induce strand breaks in DNA essentially via the formation of transient anions which decay by dissociative electron attachment (DEA) or into dissociative electronics states. The present article reports the results of a study on the influence of organic ions on this mechanism. tris and EDTA are incorporated at various concentrations within DNA films of different thicknesses. The amino group of tris molecules and the carboxylic acid function of ethylenediamine tetra-acetic acid (EDTA) molecules together can be taken as simple model for the amino acids components of proteins, such as histones, which are intimately associated with the DNA of eukaryotic cells. The yield of single strand breaks induced by 10 eV electrons is found to decrease dramatically as a function of the number of organic ions/nucleotide. As few as 2 organic ions/nucleotide are sufficient to decrease the yield of single strand breaks by 70%. This effect is partly explained by an increase in multiple inelastic electrons scattering with film thickness but changes in the resonance parameters can also contribute to DNA protection. This can occur if the electron captures cross section and the lifetime of the transient anions (i.e., core-excited resonances) formed at 10 eV are reduced by the presence of organic ions within the grooves of DNA. Moreover, it is proposed that the tris molecules may participate in the repair of DNA anions [such as G(-H)(-)] induced by DEA on DNA bases.
The Journal of chemical physics 01/2010; 132(4):045102. · 3.09 Impact Factor
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ABSTRACT: We have developed a novel apparatus that allows us to irradiate nonvolatile organic films of high mass (1-100 microg range) spread out over a large surface area (42 cm(2)) with low energy (kT-100 eV) heavy ions and to quantitatively analyze the film substance via standard biochemical techniques afterwards. Here we discuss the details of the apparatus and method and show that it allows us to measure substantial damage to double stranded DNA molecules (plasmids) and its fundamental subunits induced by heavy ions with unprecedented low energies, i.e., 2.5 eV/amu; these energies correspond to track end energies of stopping ions or secondary ions created along primary ion tracks. We find that hyperthermal Ar(+) ions interacting with plasmid DNA will lead to the formation of single and double strand breaks, as well as fragmentation of nucleosides, which also involve chemical modifications and site specific rupture along the N1-C1 glycosidic bond, resulting in base release. In cells, such localized clustered damage will enhance the severity of DNA strand lesions, thus making them harder to repair.
Review of Scientific Instruments 09/2007; 78(8):085111. · 1.37 Impact Factor
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ABSTRACT: The importance of heavy ions in radiobiology is twofold: (1) they
represent the most efficient and volume selective mode of radiotherapy
of deep-seated and non-operable tumors, (2) in space environments, or at
supersonic altitudes, the most lethal radiation consists of cosmic rays
which have a high efficiency to induce clustered DNA lesions, mutations,
and cancer. Thus, the study of their effects on DNA is essential for
radiation risk assessment, dosimetry, and efficient use of
hadrontherapy. Here, we investigate damage to DNA and its components,
induced by heavy ion impact, via a novel ion-plasma method, which allows
us to probe ion energy depositions in the 0.1-100 eV/nm range in
nanoscopic biomolecular films. Cations are generated by electron impact
in ultra pure gases (Ar, N2, CO, etc.), and are uniformly accelerated by
grids towards the inside surface of a cylinder where an organic film was
deposited. After ion irradiation at a specific energy and ion dose, the
film is recovered and analyzed. For DNA, gel electrophoresis is used to
quantify yields of single, double, and multiple strand breaks. For DNA
components (mononucleotides), fragmentation and new products are
measured by HPLC and MS.
02/2004; -1:1072.
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ABSTRACT: Electron-stimulated desorption of anions from thin films of linear and supercoiled DNA is investigated in the range 3-20 eV. Resonant structures are observed with maxima at 9.4+/-0.3, 9.2+/-0.3, and 9.2+/-0.3 eV, respectively, in the yield dependence of H-, O-, and OH- on the incident electron energy. Their formation is attributed to dissociative electron attachment.
Physical Review Letters 06/2003; 90(20):208102. · 7.37 Impact Factor
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ABSTRACT: We report direct measurements of the formation of single-, double- and multiple strand breaks in pure plasmid DNA as a function of exposure to 10-50 eV electrons. The effective cross sections to produce these different types of DNA strand breaks were determined and were found to range from approximately 10(-17) to 3 x 10(-15) cm(2). The total effective cross section and the effective range for destruction of supercoiled DNA extend from 3.4 to 4.4 x 10(-15) cm(2) and 12 to 14 nm, respectively, over the range 10-50 eV. The variation of the effective cross sections with electron energy is discussed in terms of the electron's inelastic mean free path, penetration depth, and dissociation mechanisms, including resonant electron capture; the latter is found to dominate the effective cross sections for single- and double-strand breaks at 10 eV. The most striking observations are that (1) supercoiled DNA is approximately one order of magnitude more sensitive to the formation of double-strand breaks by low-energy electrons than is relaxed circular DNA, and (2) the dependence of the effective cross sections on the incident electron energy is unrelated to the corresponding ionization cross sections. This finding suggests that the traditional notion that radiobiological damage is related to the number of ionization events would not apply at very low energies.
Radiation Research 04/2002; 157(3):227-34. · 2.68 Impact Factor
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ABSTRACT: To investigate the induction of DNA strand breaks by electrons with energies ranging from 0.1 to 1.5 keV.
Dry supercoiled plasmid DNA was irradiated with electrons of energies ranging from 0.1 to 1.5 keV and the results were compared with those obtained by gamma-irradiation of the same plasmid in solution. For electron irradiation, the plasmid was deposited on a gold substrate under a controlled atmosphere to minimize contamination of the DNA film. Electron bombardments were performed under ultra-high vacuum conditions (UHV 10(-9) torr). DNA damage was detected by gel electrophoresis followed by quantitation of the DNA bands by fluorescence or by hybridization with a radioactive probe.
Electrons with energies from 0.1 to 1.5 keV induced single, double and multiple double-strand breaks in supercoiled plasmid DNA. For equal doses, we observed a marked increase in the efficiency of induction of double- and multiple-strand breaks in supercoiled DNA as a function of electron energy. In contrast to gamma-irradiation, the formation of small DNA fragments by electrons did not seem to be related to the production of the linear form of the plasmid.
Electrons within the energy; range of the secondary electrons generated by high-energy ionizing radiation induce single, double and multiple double-strand breaks in DNA. Problems associated with low-energy electron irradiation experiments and dose calculations in thin films are also discussed.
International Journal of Radiation Biology 10/2000; 76(9):1209-21. · 2.28 Impact Factor
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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.
Science 04/2000; 287(5458):1658-60. · 31.20 Impact Factor
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ABSTRACT: Development of drug resistance is a major factor that limits the effectiveness of chemotherapy treatments. In this study, we determined whether estradiol or its metabolites 2-, 4- and 16alpha-hydroxyestrone could enhance the development of methotrexate resistance in the breast carcinoma cell line, MCF-7. Cells were incubated with the estrogens at a concentration of 10(-8) M for 12 cell doublings and enhancement of methotrexate resistance was measured with the Luria-Delbrück assay. The most efficient estrogens were the 4-hydroxyestrone and 16alpha-hydroxyestrone, which both stimulated methotrexate resistance by 88-fold as compared with the control without estrogen. 2-Hydroxyestrone had an enhancement factor of 33-fold, whereas estradiol showed a slight effect with an enhancement factor of 3.2-fold. To determine whether the estrogen receptor was involved in the development of resistance, expression of the pS2 gene, which contains an estrogen-responsive element, was measured. Both estradiol and 16alpha-hydroxyestrone stimulated expression of the pS2 gene. In contrast, 2- and 4-hydroxyestrone did not increase the level of pS2 mRNA. This suggests that tumors classified as estrogen receptor negative could also develop methotrexate resistance as the result of exposure to estrogens. The status of the tumor suppressor gene p53 was analyzed in methotrexate sensitive and resistant clones. In all the methotrexate resistant clones analyzed, the western blots indicated that the p53 protein was still present and transcriptionally competent, as measured by its capacity to stimulate transcription of the p21waf1/cip1 gene following UVB irradiation. However, the basal level of p53 was higher in resistant clones and addition of 2- or 4-hydroxyestrone increased p53 to levels equivalent to those observed following UVB irradiation. However, this induction of p53 accumulation by estrogens failed to stimulate the transcription of p21waf1/cip1, which indicates that a transcriptionally inactive form of p53 accumulated in methotrexate resistant cells.
Carcinogenesis 10/1998; 19(9):1545-52. · 5.70 Impact Factor
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ABSTRACT: Low-energy electron impact (Ei = 0−17 eV) on organic monolayers chemically bound to Au substrates is shown to induce the desorption of H2. The threshold and the maximum yields for this excitation/reaction/desorption channel are observed at Ei = 7.0 ± 0.5 eV and Ei = 10.0 ± 0.5 eV, respectively, for each of the n-alkane monolayers examined (Au−S−(CH2)nCH3, n = 3, 7, 11, and 15). From the dependence of the H2 yields on the incident electron energy in the Ei = 6−12 eV regime, which closely resemble the H- desorption yield previously reported for physisorbed alkanes, we propose that most of the H2 production originates from the dissociative electron attachment to the n-alkane film constituents; H2 formation may also proceed by direct excitation of the hydrocarbons to the dissociative S1 state. The desorption of H2 from chemisorbed Au−S−CH2C6H5 is first observed at Ei 7.5 ± 0.5 eV, with a broad maximum desorption yield extending from Ei = 12−17 eV. The sensitivity of the desorption yields to the film constituents suggests that H2 formation proceeds by molecule-specific channels and suggests that chemisorbing volatile species to conductive substrates may be useful in the study of electron-induced reactions of adsorbates at ambient or elevated temperatures. It also indicates that the use of electron beams to prepare reactive surfaces will require detailed characterization studies and highly selective excitation mechanisms to avoid undesirable decomposition channels.
03/1996;
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ABSTRACT: alpha-Amanitin, an inhibitor of RNA polymerase II, has little effect on either UV-induced incision or repair synthesis in cultured normal human fibroblasts but almost completely inhibits both processes in xeroderma pigmentosum group C fibroblasts. Cycloheximide, at a concentration which inhibits protein synthesis by 75-80%, has no effect on incision or repair synthesis in either cell type, which argues that the effects of alpha-amanitin on repair occur at the level of transcription. Cot analysis demonstrates that UV-induced repair synthesis occurs at similar levels in highly repetitive, middle repetitive and single copy sequence in both normal and xeroderma group C cells. We conclude that normal cells must have at least two excision repair pathways for repair of UV-induced damage, one dependent on transcription and the other independent.
Mutation Research/Fundamental and Molecular Mechanisms of Mutagenesis 07/1992; 274(1):57-64. · 2.85 Impact Factor
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