Showing papers by "Léon Sanche published in 2006"

DNA Damage Induced by Low-Energy Electrons: Electron Transfer and Diffraction

Abstract: Thin films of the short single strand of DNA, GCAT, in which guanine (G) or adenine (A) have been removed, were bombarded under vacuum by 4 to 15 eV electrons. The fragments corresponding to base release and strand breaks (SB) were analyzed by high performance liquid chromatography and their yields compared with those obtained from unmodified GCAT. From such a comparison, it is shown that, using GCAT as a model system, (1) most SB result from electron capture by DNA bases followed by electron transfer to the phosphate group and (2) the initial capture probability depends on the coherence of the electron wave within the tetramer.

Effective Cross Sections for Production of Single-Strand Breaks in Plasmid DNA by 0.1 to 4.7 eV Electrons

Abstract: Panajotovic, R., Martin, F., Cloutier, P., Hunting, D. and Sanche, L. Effective Cross Sections for Production of Single-Strand Breaks in Plasmid DNA by 0.1 to 4.7 eV Electrons. Radiat. Res. 165, 452–459 (2006). We determined effective cross sections for production of single-strand breaks (SSBs) in plasmid DNA [pGEM 3Zf(-)] by electrons of 10 eV and energies between 0.1 and 4.7 eV. After purification and lyophilization on a chemically clean tantalum foil, dry plasmid DNA samples were transferred into a high-vacuum chamber and bombarded by a monoenergetic electron beam. The amount of the circular relaxed DNA in the samples was separated from undamaged molecules and quantified using agarose gel electrophoresis. The effective cross sections were derived from the slope of the yield as a function of exposure and had values in the range of 10−15– 10−14 cm2, giving an effective cross section of the order of 10−18 cm2 per nucleotide. Their strong variation with incident electron energy and the resonant en...

Low-energy electron collisions with tetrahydrofuran

Abstract: We have performed calculations for electron collisions with tetrahydrofuran (THF) using the UK molecular R-matrix codes. This is the largest molecule ever treated with the R-matrix method, and the only biologically relevant molecule of this size studied theoretically in the inelastic regime. We report ab initio integral cross section for incident energies up to 10 eV. No shape resonances have been found for this system, but a few core-excited resonances are present.

Phosphodiester and N-glycosidic bond cleavage in DNA induced by 4-15 eV electrons.

Abstract: Thin molecular films of the short single strand of DNA, GCAT, were bombarded under vacuum by electrons with energies between 4 and 15 eV Ex vacuo analysis by high-pressure liquid chromatography of the samples exposed to the electron beam revealed the formation of a multitude of products Among these, 12 fragments of GCAT were identified by comparison with reference compounds and their yields were measured as a function of electron energy For all energies, scission of the backbone gave nonmodified fragments containing a terminal phosphate, with negligible amounts of fragments without the phosphate group This indicates that phosphodiester bond cleavage by 4–15 eV electrons involves cleavage of the C–O bond rather than the P–O bond The yield functions exhibit maxima at 6 and 10–12 eV, which are interpreted as due to the formation of transient anions leading to fragmentation Below 15 eV, these resonances dominate bond dissociation processes All four nonmodified bases are released from the tetramer, by c

Base Release in Nucleosides Induced by Low-Energy Electrons: A DFT Study

Abstract: Li, X., Sanche, L. and Sevilla, M.D Base Release in Nucleosides Induced by Low-Energy Electrons: A DFT Study. Radiat. Res. 165, 721–729 (2006). Low-energy electrons are known to induce strand breaks and base damage in DNA and RNA through fragmentation of molecular bonding. Recently the glycosidic bond cleavage of nucleosides by low-energy electrons has been reported. These experimental results call for a theoretical investigation of the strength of the C1′–N link in nucleosides (dA, dC and dT) between the base and deoxyribose before and after electron attachment. Through density functional theory (DFT) calculations, we compare the C1′–N bond strength, i.e., the bond dissociation energy of the neutral and its anionic radical, and find that an excess electron effectively weakens the C1′– N bond strength in nucleosides by 61–75 kcal/mol in the gas phase and 76–83 kcal/mol in the solvated environment. As a result, electron-induced fragmentation of the C1′–N bond in the gas phase is exergonic for dA (...

On the mechanism of anion desorption from DNA induced by low energy electrons.

Abstract: Our knowledge of the mechanisms of radiation damage to DNA induced by secondary electrons is still very limited, mainly due to the large sizes of the system involved and the complexity of the interactions. To reduce the problem to its simplest form, we investigated specific electron interactions with one of the most simple model system of DNA, an oligonucleotide tetrameter compound of the four bases. We report anion desorption yields from a thin solid film of the oligonucleotide GCAT induced by the impact of 3-15 eV electrons. All observed anions (H-, O-, OH-, CN-, and OCN-) are produced by dissociative electron attachment to the molecule, which results in desorption peaks between 6 and 12 eV. Above 14 eV nonresonant dipolar dissociation dominates the desorption yields. By comparing the shapes and relative intensities of the anion yield functions from GCAT physisorbed on a tantalum substrate with those obtained from isolated DNA basic subunits (i.e., bases, deoxyribose, and phosphate groups) from either the gas phase or condensed phase experiments, it is possible to obtain more details on the mechanisms involved in low energy electron damage to DNA, particularly on those producing single strand breaks.

Electron stimulated desorption of anionic fragments from films of pure and electron-irradiated thiophene

Abstract: The electron stimulated desorption (ESD) of anions is used to explore the effects of electron irradiation on a thiophene film and we report measurements for electron impact on multilayer thiophene condensed on a polycrystalline platinum substrate. Below 22eV and at low electron dose, desorbed anions include H− (the dominant signal) as well as S−, CH2−, SH− and SCH2−. Yield functions show that anions are desorbed both by dissociative electron attachment (DEA) with resonances observed at 9.5, 11, and 16eV, and for energies >13eV, by dipolar dissociation (DD). An increase in the S− signal from electron irradiated (beam-damaged) thiophene films and the appearance of a new DEA resonance in the S− yield function at 6eV are linked to rupture of the thiophene ring and the formation of sulfur-terminated products within the film. The threshold energy for ring rupture is 5eV. The desorption of new anions such as C4H3S− (Thiophene-H)−is also observed from electron irradiated films and these likely arise from the deco...

Enhanced DNA Damage Induced by Secondary Electron Emission from a Tantalum Surface Exposed to Soft X Rays

Abstract: Cai, Z., Cloutier, P., Hunting, D. and Sanche, L. Enhanced DNA Damage Induced by Secondary Electron Emission from a Tantalum Surface Exposed to Soft X Rays. Radiat. Res. 165, 365–371 (2006). Both thick and thin films of pGEM®-3Zf(-) plasmid DNA deposited on a tantalum foil were exposed to soft X rays (effective energy of 14.8 keV) for various times in air under a relative humidity of 45% (Γ ≈ 6, where Γ is the number of water molecules per nucleotide) and 84% (Γ ≈ 21), respectively. For a thick film, the DNA damage was induced chiefly by X-ray photons. For a thin film of DNA, X-ray-induced secondary electrons emitted from the tantalum result in a substantial increase in DNA damages. Different forms of plasmid DNA were separated and quantified by agarose gel electrophoresis and laser scanning. The exposure curves for the formation of nicked circular (single-strand break, SSB), linear (double-strand break, DSB), and interduplex crosslink forms 1 and 2 were obtained for both thick and thin films of ...

Induction of strand breaks by low-energy electrons (8-68 eV) in a self-assembled monolayer of oligonucleotides: Effective cross sections and attenuation lengths

Abstract: Self-assembled monolayers of 5′‐P32-labeled 3′-thiolated oligonucleotides chemisorbed on gold were bombarded by low-energy electrons (LEE) of 8–68eV. Shorter 5′‐P32-oligonucleotides produced by LEE-induced strand breaks were separated with denaturing polyacrylamide gel electrophoresis and quantified by phosphor imaging. The yields of short oligonucleotides (y) decrease exponentially with their length (n), following the equation y=ae−bn, where a and b are constants, which are related to the average effective cross section per nucleotide for DNA strand break (σeff) and the attenuation length (AL=1∕b) of LEE, respectively. The AL decreases with LEE energies from 2.5±0.6nm at 8eVto0.8±0.1nm at 68eV, whereas σeff increases from (3±1)×10−18to(5.1±1.6)×10−17cm2 within the same energy range. The energy dependence of σeff shows a resonance peak of (2.8±0.9)×10−17cm2 at 18eV superimposed on a monotonically rising curve. Transient electron attachment to a σ* anion state of the deoxyribose group, followed by dipolar ...

Cross sections for electron trapping by DNA and its component subunits I: Condensed tetrahydrofuran deposited on Kr.

Abstract: We report cross sections for electron capture processes occurring in condensed tetrahydrofuran (THF) for incident electron energies in the range of 0–9eV. The charge trapping cross section for 6–9eV electrons is very small, and an upper limit of 4×10−19cm2 is estimated from our results. This latter is thus also an upper bound for the cross section for dissociative electron attachment process that is known to occur at these energies for condensed THF. At energies close to zero eV electron trapping proceeds via intermolecular stabilization. The cross section for this process is strongly dependent on the quantity of deposited THF. Since THF may model the furyl ring found in deoxyribose, these measurements indicate that this ring likely plays little role in either initiating or enhancing strand break damage via the attachment of the low energy secondary electrons produced when DNA is exposed to ionizing radiation.

Products and reaction sequences in tetrahydrofuran exposed to low-energy electrons.

Abstract: Electron-stimulated reactions in solid films of tetrahydrofuran (THF), condensed on Kr spacers deposited on a Pt substrate, or directly onto the substrate, were induced and monitored simultaneously with use of high-resolution electron-energy-loss spectroscopy in the ranges of vibrational and electronic excitations. The spectra of the molecular films obtained after a certain time of exposure to electrons at incident energies of 14 and 15.5 eV were analyzed and different products were identified. Besides an aldehyde, which is the main product, olefins, conjugated olefins, as well as CO were identified. Closer investigation of the reactions of propionaldehyde, as a model aldehyde, demonstrates that CO appears in THF as a secondary product (i.e., from the intermediate aldehyde). On the basis of the cross sections for the formation of an aldehyde from THF, of CO from propionaldehyde, and for the loss of propionaldehyde under electron impact, the reaction sequences were evaluated with the help of a kinetic model. This analysis suggests that some CO could also be formed directly from THF (i.e., without involvement of an intermediate aldehyde).

Temporary electron localization and scattering in disordered single strands of DNA

Abstract: We present a theoretical study of the effect of structural and base sequence disorders on the transport properties of nonthermal electron scattering within and from single strands of DNA. The calculations are based on our recently developed formalism to treat multiple elastic scattering from simplified pseudomolecular DNA subunits. Structural disorder is shown to increase both the elastic scattering cross section and the attachment probability on the bases at low energy. Sequence disorder, however, has no significant effect.

Absolute cross section for trapping low-energy electrons (0–18eV) in molecular films of n-hexane

Abstract: We propose an apparatus and method, based on the absorption of photon of energy ranging between 1.5 and 3.54eV, to measure the absolute cross section for trapping low-energy electrons in a molecular film deposited on a metal substrate. A simple model is described to analyze electron trapping in the film and charge release from the same film by photon excitation. Measurements of the trapping of electrons in multilayer films of n-hexane are presented for different film thicknesses, electron exposures and incident energies, and probing photon fluxes and energies. The electron trapping cross section in a five-layer film of n-hexane is characterized by a large maximum reaching a value of (4.7±0.4)×10−17cm2 at 10eV and having a 2.3eV full width at half maximum. This feature is ascribed to the formation of one or several core-excited resonances lying between 7 and 14eV and leading, via dissociative electron attachment, to the formation of stable H− fragments, which stabilize within the first outerlayer of the film.