Formation Of Transient Anions Research Articles

Absolute cross-sections for DNA strand breaks and crosslinks induced by low energy electrons.

Absolute cross sections (CSs) for the interaction of low energy electrons with condensed macromolecules are essential parameters to accurately model ionizing radiation induced reactions. To determine CSs for various conformational DNA damage induced by 2-20 eV electrons, we investigated the influence of the attenuation length (AL) and penetration factor (f) using a mathematical model. Solid films of supercoiled plasmid DNA with thicknesses of 10, 15 and 20 nm were irradiated with 4.6, 5.6, 9.6 and 14.6 eV electrons. DNA conformational changes were quantified by gel electrophoresis, and the respective yields were extrapolated from exposure-response curves. The absolute CS, AL and f values were generated by applying the model developed by Rezaee et al. The values of AL were found to lie between 11 and 16 nm with the maximum at 14.6 eV. The absolute CSs for the loss of the supercoiled (LS) configuration and production of crosslinks (CL), single strand breaks (SSB) and double strand breaks (DSB) induced by 4.6, 5.6, 9.6 and 14.6 eV electrons are obtained. The CSs for SSB are smaller, but similar to those for LS, indicating that SSB are the main conformational damage. The CSs for DSB and CL are about one order of magnitude smaller than those of LS and SSB. The value of f is found to be independent of electron energy, which allows extending the absolute CSs for these types of damage within the range 2-20 eV, from previous measurements of effective CSs. When comparison is possible, the absolute CSs are found to be in good agreement with those obtained from previous similar studies with double-stranded DNA. The high values of the absolute CSs of 4.6 and 9.6 eV provide quantitative evidence for the high efficiency of low energy electrons to induce DNA damage via the formation of transient anions.

Low-energy electron-induced dissociation in condensed-phase L-cysteine I: Desorption of anions from chemisorbed films

Among amino acids, cysteine has been widely studied, becoming a standard for molecular self-assembly experiments, because its mercapto group (-SH) allows the formation of self-assembled monolayers (SAMs) on metal surfaces. Dissociative electron attachment (DEA) on L-cysteine SAMs is investigated utilizing a time-of-flight mass spectrometer coupled with a low-energy electron gun. The results show that electrons with kinetic energies of 3 to 15 eV attach to L-cysteine producing anionic fragments of different masses (e.g., H-, O-, OH-, S-, SH-) via dissociation of intermediate transient anions. The anion yield functions exhibited purely resonant behaviour with electron energies below 15 eV, indicating that the formation of transient anions is the predominant mechanism of production of anionic fragments from L-cysteine dissociation.

Protection by organic ions against DNA damage induced by low energy electrons.

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.

Role of Secondary Low-Energy Electrons in the Concomitant Chemoradiation Therapy of Cancer

Solid films of DNA with and without the chemotherapeutic agent cisplatin bonded to guanine were bombarded with electrons of 1, 10, 100, and 60,000 eV causing single and double strand breaks. In the presence of cisplatin this damage was increased by factors varying from 1.3 to 4.4 owing to an increase in bond dissociation triggered by the formation of transient anions. This mechanism may lie at the basis of the efficiency of concomitant cisplatin-radiation therapy.

Nanoscopic aspects of radiobiological damage: Fragmentation induced by secondary low-energy electrons.

Low-energy electrons (LEEs) are produced in large quantities in any type of material irradiated by high-energy particles. In biological media, these electrons can fragment molecules and lead to the formation of highly reactive radicals and ions. The results of recent experiments performed on biomolecular films bombarded with LEEs under ultra-high vacuum conditions are reviewed in the present article. The major type of experiments, which measure fragments produced in such films as a function of incident electron energy (0.1-45 eV), are briefly described. Examples of the results obtained from DNA films are summarized along with those obtained from the fragmentation of elementary components of the DNA molecule (i.e., thin solid films of H(2)O, DNA bases, sugar analogs, and oligonucleotides) and proteins. By comparing the results of these different experiments, it is possible to determine fundamental mechanisms that are involved in the dissociation of biomolecules and the production of single- and double-strand breaks in DNA, and to show that base damage is dependent on the nature of the bases and on their sequence context. 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 usually initiate other reactions with nearby molecules, causing further chemical damage. The damage caused by transient anions is dependent on the molecular environment.

Electronic aging and related electron interactions in thin-film dielectrics

The author points out that hot electrons generated in dielectrics subjected to high electrical field strengths can produce highly reactive chemical species which contribute to the aging process. The study of the interaction of such low energy (0 to 15 eV) electrons near the surface of dielectrics in experiments combining cryogenic thin film deposition and high-resolution low-energy electron-beam techniques is discussed. Examples of the results obtained with thin film atomic and molecular solids are used to illustrate the basic mechanisms which control the electron-dielectric interactions and provide a description of the basic degradation processes involved during electronic aging. It is shown that elastic and quasi-elastic scattering of hot electrons in dielectrics can be described in terms of band structure parameters, whereas inelastic scattering is often governed by the formation of transient anions. These anions can decay by stabilization, by producing vibrationally and electronically excited molecules, or by dissociating into a stable anion and a neutral radical. These latter species usually initiate other reactions with nearby molecules, causing further chemical damage. >

Absolute vibrational excitation cross sections for slow-electron (1-18 eV) scattering in solid H2O.

High-resolution vibrational electron-energy-loss spectra of amorphous ice films condensed at 14 K are reported for the incident-energy range 1--18 eV. Absolute electron scattering cross sections for elastic collisions, individual vibrational excitations, and the sum of electronic transitions are obtained by performing a two-stream multiple-scattering analysis of the spectra. The various features found in the energy-dependent cross sections are discussed, whenever possible, by comparison with data and mechanisms (e.g., transient anion formation) well established in the gas phase. Quantum-interference effects introduced implicitly in the cross sections by the classical analysis are discussed within the first Born approximation for electron--multiple-site scattering.

Density and temperature effects on electron mobility in gaseous, critical, and liquid ethene

Contrary to previous reports, the electron scattering cross section of ethene molecules has a Ramsauer-Townsend (RT) minimum at 0.09-0.10 eV. The energy of the RT minimum is much higher than that for nitrogen, slightly lower than that for ethane, and much lower than those for methane and xenon. The scattering of low-energy (${10}^{\ensuremath{-}3}$-1 eV) electrons by nonpolar molecules in the low-density gas phase is strongly influenced by the molecular shape. The mobility of electrons (the proportionate rate of motion in the direction of an applied electric field) in the low-density gas is mainly governed by elastic collisions, but the removal of energy gained from the field occurs mainly through inelastic collisions. The latter must involve predominantly molecular rotational transitions at the energies involved (0.1 eV). In the dense gas at $\frac{n}{{n}_{c}}\ensuremath{\gtrsim}0.2$ the density-normalized mobility $\ensuremath{\mu}n$ decreases due to electron quasilocalization and transient-anion formation. The transient anions in ethene are much more ephemeral than those formed in dense nitrogen vapor. The anion state becomes less ephemeral with increasing density and decreasing temperature in the liquid phase.