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.

http://science.sciencemag.org/content/sci/287/5458/1658.full.pdf

Resonant Formation of DNA Strand Breaks by Low-Energy (3 to 20 eV) Electrons

Tetrathiafulvalenes, oligoacenenes, and their buckminsterfullerene derivatives: the brick and mortar of organic electronics.

On the basis of different types of experiments, the authors develop implicitly the model of surface-enhanced Raman scattering (SERS) of adsorbates on metal surfaces. The long-range enhancement by resonances of the macroscopic laser and Stokes field is separated quantitatively from the metal electron-mediated resonance Raman effect. The latter mechanism proceeds by increased electron-photon coupling at an atomically rough surface and by temporary charge transfer to orbitals of the adsorbates. This model can account for the chemical specificity and vibrational selectivity of SERS and (partly) for the SERS specificity of the various metals.

https://iopscience.iop.org/article/10.1088/0953-8984/4/5/001/pdf

Surface-enhanced Raman scattering

The reactive force-field (ReaxFF) interatomic potential is a powerful computational tool for exploring, developing and optimizing material properties. Methods based on the principles of quantum mechanics (QM), while offering valuable theoretical guidance at the electronic level, are often too computationally intense for simulations that consider the full dynamic evolution of a system. Alternatively, empirical interatomic potentials that are based on classical principles require significantly fewer computational resources, which enables simulations to better describe dynamic processes over longer timeframes and on larger scales. Such methods, however, typically require a predefined connectivity between atoms, precluding simulations that involve reactive events. The ReaxFF method was developed to help bridge this gap. Approaching the gap from the classical side, ReaxFF casts the empirical interatomic potential within a bond-order formalism, thus implicitly describing chemical bonding without expensive QM calculations. This article provides an overview of the development, application, and future directions of the ReaxFF method.

https://biblio.ugent.be/publication/8514189/file/8514192.pdf

The ReaxFF reactive force-field: development, applications and future directions

I. Introduction and Scope 231A. Definitions of Atomic Electron Affinities 233B. Definitions of Molecular Electron Affinities 233II. Experimental Photoelectron Electron Affinities 235A. Historical Background 235B. The Photoeffect 236C. Experimental Methods 237D. Time-of-Flight Negative Ion PhotoelectronSpectroscopy239E. Some Thermochemical Uses of ElectronAffinities241F. Layout of Table 10: ExperimentalPhotoelectron Electron Affinities242III. Theoretical Determination of Electron Affinities 242A. Historical Background 2421. Theoretical Predictions of Atomic ElectronAffinities2422. Theoretical Predictions of MolecularElectron Affinities243B. Present Status of Theoretical Electron AffinityPredictions243C. Basis Sets and Theoretical Electron Affinities 244D. Density Functional Theory (DFT) andElectron Affinities245E. Layout of Tables 8 and 9: Theoretical DFTElectron Affinities247F. Details of Density Functional MethodsEmployed in Tables 8 and 9247IV. Discussion and Observations 248A. Statistical Analysis of DFT Results ThroughComparisons to Experiment and OtherTheoretical Methods248B. Theoretical EAs for Species with UnknownExperimental EAs251C. On the Applicability of DFT to Anions and theFuture of DFT EA Predictions251D. Specific Theoretical Successes 251E. Interesting Problems 2521. C

/pdf/atomic-and-molecular-electron-affinities-photoelectron-3x4vklpj6q.pdf

Atomic and molecular electron affinities: photoelectron experiments and theoretical computations.

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.

Injection of electrons into the empty π* molecular orbitals of uracil and the DNA bases creates short-lived anion states whose energies have been determined by electron scattering. A common range of attachment energies into the lowest orbitals is observed in all the bases. Evidence for nuclear motion during the lifetimes of the anions is found in all the compounds except adenine. These properties of the bases as bridge sites along the π-stack of DNA, namely, the effective degeneracy of the anion energies and the strong excitation of vibration, are key parameters for theories of electron-transfer rate, some of which lead to inverse rather than exponentially decreasing bridge-length dependences.

/pdf/electron-attachment-energies-of-the-dna-bases-4eoamo03ad.pdf

Electron Attachment Energies of the DNA Bases

The experimental methods used to characterize molecular anions differ substantially depending on the stability of the species. The application of laser spectroscopic methods, discussed elsewhere in this issue, is generally limited to anions possessing hound electronic ground states or to those with lifetimes against autodetachment of the electron in excess of a few microseconds. For a great many molecules, including such important prototypes as ethylene, butadiene, benzene, naphthalene, formaldehyde, and acetylene, the ground states of the anions are known to be unstable in the gas phase with lifetimes less than lo-'* s, typically in the neighborhood of s. Even for those molecules with hound ground-state anions, nearly all of the excited anion states lie in the continuum and hence decay by electron detachment. Thus from consideration of numbers alone, the manifold of temporary anion states

/pdf/temporary-anion-states-of-polyatomic-hydrocarbons-3tebrrqm05.pdf

Temporary Anion States of Polyatomic Hydrocarbons

/pdf/studies-of-the-temporary-anion-states-of-unsaturated-56icjdl7se.pdf

Studies of the temporary anion states of unsaturated hydrocarbons by electron transmission spectroscopy

Low energy electrons are capable of breaking bonds in gas phase DNA bases by means of the dissociative electron attachment process. With the aid of new total scattering data in the halouracils and input from quantum chemical calculations, we describe the dipole bound and valence anion states in these compounds and present assignments for the two types of structure appearing in the cross sections. A clear distinction between the two mechanisms for bond breaking is necessary for an understanding of electron induced damage to DNA.

/pdf/bond-breaking-and-temporary-anion-states-in-uracil-and-3mbqq08dra.pdf

Paul Burrow

Papers

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

Electron Attachment Energies of the DNA Bases

Temporary Anion States of Polyatomic Hydrocarbons

Studies of the temporary anion states of unsaturated hydrocarbons by electron transmission spectroscopy

Bond breaking and temporary anion states in uracil and halouracils: implications for the DNA bases.