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L Sanche

Publications and source records attributed to L Sanche.

At least 37 records · Page 2Linked to original sources
PubMed2002

Alteration of protein structure induced by low-energy (<18 eV) electrons. I. The peptide and disulfide bridges.

We present measurements of low-energy (<18 eV) electron-stimulated desorption of anions from acetamide (CH(3)CONH(2)) and dimethyl disulfide [DMDS: (CH(3)S)(2)] films. Electron irradiation of physisorbed CH(3)CONH(2) produces H(-), CH(3)(-) and O(-) anions, whereas the H(-), CH(2)(-), CH(3)(-), S(-), SH(-) and SCH(3)(-) anions are observed to desorb from the DMDS film. Below 12 eV, the dependence of the anion yields on the incident electron energy exhibits structures that indicate that a resonant process (i.e. dissociative electron attachment) is responsible for molecular fragmentation. Within the range of 1-18 eV, it is found that (1.7 and 1.4) x 10(7) H(-) ions/incident electron and (7.8 x 10(-11) and 4.3 x 10(-8)) of the other ions/incident electron are desorbed from acetamide and DMDS films, respectively. These results suggest that, within proteins, the disulfide bond is more sensitive to low-energy electron attack than the peptide bond. In biological cells, some proteins interact closely with nucleic acid. Therefore, the observed fragments, when produced from secondary low-energy electrons generated by high-energy radiation, not only may denature proteins, but may also induce reactions with the nearby nucleic acid and damage DNA.

Acetamides↗
PubMed2001

Effects of cosmic rays on atmospheric chlorofluorocarbon dissociation and ozone depletion.

Data from satellite, balloon, and ground-station measurements show that ozone loss is strongly correlated with cosmic-ray ionization-rate variations with altitude, latitude, and time. Moreover, our laboratory data indicate that the dissociation induced by cosmic rays for CF(2)Cl(2) and CFCl(3) on ice surfaces in the polar stratosphere at an altitude of approximately 15 km is quite efficient, with estimated rates of 4.3 x 10(-5) and 3.6 x 10(-4) s(-1), respectively. These findings suggest that dissociation of chlorofluorocarbons by capture of electrons produced by cosmic rays and localized in polar stratospheric cloud ice may play a significant role in causing the ozone hole.

Air Pollutants↗
PubMed2001

Low-energy (5-40 eV) electron-stimulated desorption of anions from physisorbed DNA bases.

We present the results of experiments on anion desorption from the physisorbed DNA bases adenine, thymine, guanine and cytosine induced by the impact of low-energy (5-40 eV) electrons. Electron bombardment of DNA base films induces ring fragmentation and desorption of H(-), O(-), OH(-), CN(-), OCN(- ) and CH(2)(-) anions through either single or complex multibond dissociation. We designate the variation of the yield of an anion with electron energy as the yield function. Below 15 eV incident electron energy, bond cleavage is controlled mainly by dissociative electron attachment. Above 15 eV, the portion of a yield function that increases linearly is attributed to nonresonant processes, such as dipolar dissociation. A resonant structure is superimposed on this signal around 20 eV in the anion yield functions. This structure implicates dissociative electron attachment and/or resonant decay of the transient anion into the dipolar dissociation channel, with a minimal contribution from multiple inelastic electron scattering. The yields of all desorbing anions clearly show that electron resonances contribute to the damage of all DNA bases bombarded with 5-40 eV electrons. Comparison of the ion yields indicates that adenine is the least sensitive base to slow electron attack. Electron-irradiated guanine films exhibit the largest yields of desorbed anions.

Adenine↗
PubMed2001

Sequence-specific damage induced by the impact of 3-30 eV electrons on oligonucleotides.

The ability of low-energy electrons to induce single- and double-strand breaks in DNA has recently been demonstrated. Here we show the propensity of 3-30 eV electrons to initiate base sequence-dependent damage to a short single DNA strand. Solid monolayer films of homogeneous thymidine (T(9)) and deoxycytidine (dCy(9)) and heterogeneous oligomers (T(6)dCy(3)) are bombarded with 1-30 eV electrons in an ultrahigh-vacuum system. CN, OCN and/or H(2)NCN are detected by a mass spectrometer as the most intense neutral fragments desorbing in vacuum. A weaker signal of CH(3)CCO is also detected, but only from oligonucleotides containing thymine. Below 17 eV, the energy dependence of the yields of CN, OCN and CH(3)CCO exhibits resonance-like structures, attributed to dissociative electron attachment (DEA). Above 17 eV, the monotonic increase in the fragment yields indicates that nonresonant processes (i.e. dipolar dissociation) control the fragmentation of these molecules. Within the energy range investigated, comparison of the magnitude of the total fragment yields produced by electron attack on dCy(9), T(6)-dCy(3) and T(9) suggests the following order in the sensitivity of single-strand DNA: dCy(9) > T(6)-dCy(3) > T(9). At 12 eV, the total fragment yields are found to be 5.8, 5.0 and 3.9 x 10(-3) fragment/electron, respectively. From the yields obtained with the two homo-oligonucleotides, we differentiate between contributions arising from the chemical nature of the base and the effect of environment (i.e. the sequence) when a thymidine unit in T(9) is replaced by dCy. The base sequence-dependent damage is found to vary with incident electron energy. These results reinforce the idea that genomic sensitivity to ionizing radiation depends on local genetic information. Furthermore, they underscore the possible role of low-energy electrons in the pathways responsible for the induction of specific genomic lesions.

Base Sequence↗
PubMed2000

Resonant formation of DNA strand breaks by low-energy (3 to 20 eV) electrons.

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.

Chemical Phenomena↗
PubMed2000

Induction of single- and double-strand breaks in plasmid DNA by 100-1500 eV electrons.

PURPOSE: To investigate the induction of DNA strand breaks by electrons with energies ranging from 0.1 to 1.5 keV. MATERIALS AND METHODS: 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. RESULTS: 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. CONCLUSIONS: 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.

DNA↗
PubMed2000

Damage induced by 1-30 eV electrons on thymine- and bromouracil-substituted oligonucleotides.

The impact of low-energy (1-30 eV) electrons on self-assembled monolayers of heterogeneous oligonucleotides chemisorbed on a gold surface has been investigated by mass spectrometry of desorbed neutral species in an attempt to understand the consequences of secondary electron damage in a short sequence of a DNA single strand. We demonstrate that the most intense observable neutral species (CN, OCN and/or H(2)NCN) desorbed from Cy(6)-Th(3) and Cy(6)-(BrdU)(3) oligos are related to primary fragmentation of the bases induced by electron impact. The dependence of the neutral species desorption on electron energy shows typical signatures of dissociative electron attachment initiated by the formation of shape- and core-excited resonances (i.e. single-electron and two-electron- one-hole transitory anions, respectively). Substitution of dTh by BrdU increases the production of neutral fragments by as much as a factor of about 3 for the entire electron energy range. When the distribution of secondary electrons along radiation tracks in H(2)O is taken into account, we show that the probability for electron damage to heterogeneous oligonucleotides is enhanced by a factor of 2.5-3 for electron energies below 20 eV for both sensitized and unsensitized strands.

Bromouracil↗
PubMed1999

Low-energy (5-25 eV) electron damage to homo-oligonucleotides.

Radiation-induced damage to homo-oligonucleotides is investigated by electron-stimulated desorption of neutral fragments from chemisorbed organic films. Six and 12 mers of cytidine phosphate (poly dCs) and thymidine phosphate (poly dTs) are chemisorbed from various solutions onto a crystalline gold substrate by a thiol modification at the 3' end and are irradiated under ultra-high vacuum conditions with 5-25 eV electrons. The mass selected neutral desorption yields consist mainly of fragments of the DNA bases, i.e. CN and OCN (and/or H2NCN for poly dCs) from both poly dCs and poly dTs, indicating that the electrons interact specifically via fragmentation of the aromatic ring of either of the bases. Other heavier fragments are also detected such as H3CC-CO from poly dTs. The yields generally possess a threshold near 5 eV and a broad maximum around 12-13 eV incident electron energy. Dissociative electron attachment as well as electronically excited neutral or cation states are believed to be responsible for the various desorption yields. The latter yields are consistently larger for oligos chemisorbed from water and acetone solutions, compared to methanol solution. The invariance of the fragment yield intensities with oligo length suggests that the molecules are likely to adsorb almost parallel to the surface.

DNA Damage↗
PubMed1999

Halogen anion formation in 5-halouracil films: X rays compared to subionization electrons.

The radiosensitization properties of 5-halouracils (5-FU, 5-BrU and 5-IU), i.e. the enhanced sensitivity of biological media containing these compounds to ionizing radiation, have been studied using surface science methods. We show that soft X rays and near 0 eV electrons both induce dissociation of 5-halouracils into a halogen anion and a uracilyl radical. The yield of anions from 5-FU is much smaller than that from the bromo- and iodo-analogs. We explain the high anion yields in 5-BrU and 5-IU with dissociative electron attachment (DEA) of near 0 eV electrons. The thermodynamic threshold for DEA to 5-FU is near 2 eV and therefore prohibits dissociation by near 0 eV electrons.

Anions↗
PubMed1998

Absolute and effective cross-sections for low-energy electron-scattering processes within condensed matter.

Within the last two decades, a number of experimental techniques have been developed to measure mean free paths and absolute and effective cross-sections for various processes related to the interaction of low-energy electrons with condensed matter. In all of the experiments, a monochromatic electron beam impinges on a thin multilayer film composed of atoms and/or molecules condensed on a metal or semiconductor substrate held at cryogenic temperatures in an ultra-high-vacuum system. Depending on the apparatus, cross-sections are obtained from low-energy electron transmission (LEET), high-resolution electron energy loss (HREEL), x-ray photoelectron (XPS) spectroscopy, electron-stimulated desorption (ESD) of neutral and ions, or a combination of these techniques. Quasi-elastic and inelastic mean free paths have been extracted from LEET data. This method has also served to generate absolute cross-sections for electron trapping and fragment production from the dissociation of transient molecular anions. In amorphous ice, a complete set of absolute cross-sections for all inelastic losses by 1-20 eV electrons has been obtained from HREEL data. Effective cross-sections for neutral and ionic radical formation were generated by desorption and XPS experiments. These various methods are briefly described in this article, and the corresponding cross-sections in the range 0-20 eV summarized.

Argon↗