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.

/pdf/low-energy-electron-driven-damage-in-biomolecules-5cn6qjrfa5.pdf

Low energy electron-driven damage in biomolecules

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.

/pdf/dna-strand-breaks-induced-by-0-4-ev-electrons-the-role-of-1ngkm1xm4r.pdf

DNA strand breaks induced by 0-4 eV electrons: the role of shape resonances.

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Precursors of solvated electrons in radiobiological physics and chemistry.

At the very early time of irradiation, ballistic secondary electrons are produced as the most abundant of the radiolytic species directly within DNA or its environment. Here, we demonstrate the propensity of such low-energy (<3 eV) electrons to damage DNA bases via an effective loss of hydrogen located at the specific nitrogen positions. Since this site is directly implicated in the bonding of nucleobases within DNA and since dehydrogenation of the nucleic acid bases has been observed to be the predominant dissociative channel, the present findings foreshadow significant implications for the initial molecular processes leading to genotoxicity in living cells following unwanted or intended exposure to ionizing radiation (e.g., sunbathing, air travel, radiotherapy, etc.).

Site-specific dissociation of DNA bases by slow electrons at early stages of irradiation.

Electron-induced damage of DNA and its components: Experiments and theoretical models

The interaction of high energy radiation (� , � , � rays or heavy ions) with living cells does not in general directly lead to DNA strand breaks. The primary interaction essentially removes electrons from the components of the complex molecular network, i.e., electrons from valence states of the chemical bonds but also electrons from localized inner shells of the individual atoms. As a result of the subsequent charge transfer and energy dissipating processes, chemical bonds can be ruptured generating neutral or ionic radicals as additional secondary species. Electrons as the most abundant secondary species are created with an estimated quantity of � 4 � 10 4 electrons per MeV primary quantum deposited [1]. The larger majority possesses initial kinetic energies up to about 20 eV [2]. In the course of successive inelastic collisions within the medium they are thermalized within 10 � 12 s before they reach some stage of solvation, then as chemical rather inactive species. Moreover, damage of the genom in a living cell by ionizing radiation is about one-third direct and two-thirds indirect [3]. Direct damage concerns reactions directly in the DNA and its closely bound water molecules and indirect damage results from energy deposition in water molecules and other biomolecules in the surrounding of the DNA. It is believed that almost all the indirect damage is due to the attack of the highly reactive hydroxyl radical [4,5]. The importance of reactions of presolvated electrons with amino acids and nucleotides has already been pointed out more than two decades ago by time resolved pulse radiolysis experiments [6]. More recently, the ability of free ballistic electrons (3‐20 eV) to efficiently induce single and double strand breaks in supercoiled DNA has clearly been shown by Sanche and co-workers [7]. In these studies it was demonstrated that the DNA strand breaks were initiated by the formation and decay of transient negative ion (TNI) states, localized on the various DNA components (base, phosphate, deoxyribose, or hydration water). Resonances in DNA strand break curves were observed in the energy range around 10 eV ,s imilar to those TNI states formed by these components in the gas phase or in homogeneous films as exemplified in Ref. [7]

/pdf/electron-attachment-to-uracil-effective-destruction-at-3npfuey7fa.pdf

Electron attachment to uracil: effective destruction at subexcitation energies.

We present results about dissociative electron attachment (DEA) to gas-phase uracil (U) for incident electron energies between 0 and 14 eV using a crossed electron/molecule beam apparatus. The most abundant negative ion formed via DEA is (U-H)−, where the resonance with the highest intensity appears at 1.01 eV. The anion yield of (U-H)− shows a number of peaks, which can be explained in part as being due to the formation of different (U-H)− isomers. Our results are compared with high level ab initio calculations using the G2MP2 method. There was no measurable amount of a parent ion U−. We also report the occurrence of 12 other fragments produced by dissociative electron attachment to uracil but with lower cross sections than (U-H)−. In addition we observed a parasitic contaminating process for conditions where uracil was introduced simultaneously with calibrant gases SF6 and CCl4 that leads to a sharp peak in the (U-H)− cross section close to 0 eV. For (U-H)− and all other fragments we determined rough me...

Electron attachment to gas-phase uracil.

Ionization and fragmentation of water and uracil molecules was studied both by electron and proton impact. A special coincidence technique allows on an event by event basis the investigation of product ions formed upon the collision of protons with neutral molecules including the identification of the charge state of the projectile. This enables the characterization of the ionization processes occurring, i.e. direct ionization, single electron capture or double electron capture for 0, 1 or 2 electrons that are transferred from the target to the projectile, respectively. For uracil the fragmentation patterns obtained by electron and proton impact ionization reveal close similarities and indicate a comparable amount of excitation for the two different ionization mechanisms at high enough projectile energies.

/pdf/inelastic-interactions-of-protons-and-electrons-with-32wdljglce.pdf

Inelastic interactions of protons and electrons with biologically relevant molecules

Electron attachment (EA) and dissociative electron attachment (DEA) to 5-chloro uracil (5-ClU) was studied in the gas phase using a crossed electron/molecule beams technique. Besides production of a parent anion via a zero energy resonance, ion yields of nine different negative ions were observed in the electron energy range from about 0 to 14 eV. In the electron energy range from about zero to 5 eV, the formation of a transient negative ion was induced by electron attachment to the π* resonances located at about 0.24, 1.5, and 3.6 eV leading subsequently by unimolecular decay to various negative fragment ions. Absolute partial cross sections for EA and DEA to 5-ClU were obtained from the measured ion yields using a simple calibrating method. The dominant negative ion observed in the present experiment was (C4H2N2O2)− (corresponding to 5-ClU minus HCl) with a mass to charge ratio of 110, followed by Cl− ion (mass to charge ratios 35 and 37), the partial cross sections being σ(0.23 eV)=5×10−18 m2 and σ(0.23 eV)=3×10−18 m2, respectively. The parent anion produced (5-ClU)− has only a cross section value of σ(0 eV)=3×10−20 m2. The energetic thresholds for the formation of particular negative ions from 5-ClU in the gas phase were calculated at the G2(MP2) level of theory and compared with the experimental results. On the basis of these calculations structure and relative stability of some of the fragment ions was predicted. In the case of the two most abundant ions, their formation was observed well below the calculated electron energy threshold. The formation of these negative ions below the thermodynamic threshold is explained in terms of the high vibrational energy of the 5-ClU prior to the EA process.

Electron attachment to 5-chloro uracil

http://www.geology.wisc.edu/~johnf/g777/777Penelope/Deutsch1.pdf

B. Gstir

Papers

Electron attachment to uracil: effective destruction at subexcitation energies.

Electron attachment to gas-phase uracil.

Inelastic interactions of protons and electrons with biologically relevant molecules

Electron attachment to 5-chloro uracil

Calculated electron impact cross sections for the K-shell ionization of Fe, Co, Mn, Ti, Zn, Nb, and Mo atoms using the DM formalism