O A Fojón

Quantum-mechanical predictions of DNA and RNA ionization by energetic proton beams

Among the numerous constituents of eukaryotic cells, the DNA macromolecule is considered as the most important critical target for radiation-induced damages. However, up to now ion-induced collisions on DNA components remain scarcely approached and theoretical support is still lacking for describing the main ionizing processes. In this context, we here report a theoretical description of the proton-induced ionization of the DNA and RNA bases as well as the sugar-phosphate backbone. Two different quantum-mechanical models are proposed: the first one based on a continuum distorted wave-eikonal initial state treatment and the second perturbative one developed within the first Born approximation with correct boundary conditions (CB1). Besides, the molecular structure information of the biological targets studied here was determined by ab initio calculations with the Gaussian 09 software at the restricted Hartree-Fock level of theory with geometry optimization. Doubly, singly differential and total ionization cross sections also provided by the two models were compared for a large range of incident and ejection energies and a very good agreement was observed for all the configurations investigated. Finally, in comparison with the rare experiment, we have noted a large underestimation of the total ionization cross sections of uracil impacted by 80 keV protons,whereas a very good agreement was shown with the recently reported ionization cross sections for protons on adenine, at both the differential and the total scale.

Theoretical predictions for ionization cross sections of DNA nucleobases impacted by light ions.

Induction of DNA double strand breaks after irradiation is considered of prime importance for producing radio-induced cellular death or injury. However, up to now ion-induced collisions on DNA bases remain essentially experimentally approached and a theoretical model for cross section calculation is still lacking. Under these conditions, we here propose a quantum mechanical description of the ionization process induced by light bare ions on DNA bases. Theoretical predictions in terms of differential and total cross sections for proton, α-particle and bare ion carbon beams impacting on adenine, cytosine, thymine and guanine bases are then reported in the 10 keV amu(-1)-10 MeV amu(-1) energy range. The calculations are performed within the first-order Born approximation (FBA) with biological targets described at the restricted Hartree-Fock level with geometry optimization. Comparisons to recent theoretical data for collisions between protons and cytosine point out huge discrepancies in terms of differential as well as total cross sections whereas very good agreement is shown with our previous classical predictions, especially at high impact energies (E(i) ≥ 100 keV amu(-1)). Finally, in comparison to the rare existing experimental data a systematic underestimation is observed in particular for adenine and thymine whereas a good agreement is reported for cytosine. Thus, further improvements appear as necessary, in particular by using higher order theories like the continuum-distorted-wave one in order to obtain a better understanding of the underlying physics involved in such ion-DNA reactions.

Proton-induced single electron capture on DNA/RNA bases.

In this work, we report total cross sections for the single electron capture process induced on DNA/RNA bases by high-energy protons. The calculations are performed within both the continuum distorted wave and the continuum distorted wave-eikonal initial state approximations. The biological targets are described within the framework of self-consistent methods based on the complete neglect of differential overlap model whose accuracy has first been checked for simpler bio-molecules such as water vapour. Furthermore, the multi-electronic problem investigated here is reduced to a mono-electronic one using a version of the independent electron approximation. Finally, the obtained theoretical predictions are confronted with the scarcely available experimental results.

Interference effects in single ionization of molecular hydrogen by electron impact

A recently developed molecular three-continuum approximation is employed to compute differential cross sections for the ionization of hydrogen molecules by electron impact. Within the framework of this approximation, the chosen final electronic wavefunction takes into account the molecular character of the target as well as the correlate motion between the aggregates in the final channel of the reaction. Fivefold-differential cross sections as a function of both the electron momenta in the final state and the molecular orientation are studied for different kinematical arrangements. Interference structures coming from the two-centre geometry of the molecule are predicted in this case. Integrated cross sections over all molecular orientations are also calculated. It is shown that interference patterns remain, even for this case.

Influence of the dynamic screening on single-electron ionization of multi-electron atoms

A complete formulation of the post-version of the continuum distorted wave-eikonal initial state model to investigate single-electron ionization of multi-electron atoms by fast bare ion beams is considered. The influence of the non-ionized electrons on the dynamic evolution of the ejected electron is analysed showing that the corresponding interaction plays a main role in the determination of double differential cross sections. It is demonstrated that its inclusion as an additional term in the perturbative potential of the exit channel avoids discrepancies between the pre- and post-versions of the studied distorted wave model.

Water versus DNA: new insights into proton track-structure modelling in radiobiology and radiotherapy.

Water is a common surrogate of DNA for modelling the charged particle-induced ionizing processes in living tissue exposed to radiations. The present study aims at scrutinizing the validity of this approximation and then revealing new insights into proton-induced energy transfers by a comparative analysis between water and realistic biological medium. In this context, a self-consistent quantum mechanical modelling of the ionization and electron capture processes is reported within the continuum distorted wave-eikonal initial state framework for both isolated water molecules and DNA components impacted by proton beams. Their respective probability of occurrence-expressed in terms of total cross sections-as well as their energetic signature (potential and kinetic) are assessed in order to clearly emphasize the differences existing between realistic building blocks of living matter and the controverted water-medium surrogate. Consequences in radiobiology and radiotherapy will be discussed in particular in view of treatment planning refinement aiming at better radiotherapy strategies.

Positronium formation in collisions of fast positrons impacting on vapour water molecules

Positronium formation through electron capture by fast positrons impinging on vapour water molecules is studied theoretically at intermediate and high impact energies. This multi-electron system is treated within the framework of the independent electron model. The charge transfer process is described by employing the continuum distorted-wave final-state approximation. In this model, the final state of the collision is distorted by two Coulomb wavefunctions associated with the interactions of both the positron and the active electron (the captured one) with the residual ionic target. The water molecule is described by an expansion of monocentric Slater-like functions centred on the oxygen atom. Total cross sections are computed by using a partial-wave technique and compared with results obtained using the simpler Coulomb– Born approximation. Theoretical results for the production of H2O + ions in the final channel of the reaction through charge transfer processes (positronium formation and ionization) are also presented.

Fast-electron impact ionization of molecular hydrogen: energy and angular distribution of double and single differential cross sections and Young-type interference

We report the energy and angular distribution of absolute double differential cross sections (DDCSs) of ejected electrons in collisions of 8 keV projectile electrons with molecular hydrogen. The ejected electrons with energy between 1 eV and 400 eV and ejection angles between 30° and 150° are detected. The measured data are compared with the theoretical calculations based on two-effective centre (TEC) model. The first-order interference is derived from the energy distribution of DDCS and the resulting ratio spectra (H2 to 2H) exhibit oscillating behaviour. The signature of first-order interference is also demonstrated in the DDCS spectra as a function of the ejection angle. We have shown that the constructive interference prevails in soft- and binary-collision regions. The single differential cross sections (SDCS) are deduced by integrating the DDCS over the solid angle as well as ejection energy. We demonstrate that the SDCS and corresponding ratio spectra also preserve the signature of interference.

Ionization of helium targets by proton impact: a four-body distorted wave-eikonal initial state model and electron dynamic correlation

Single ionization of dielectronic atomic targets by the impact of protons is theoretically investigated. To describe this process, a four-body distorted wave model is proposed where both electrons are considered as active ones. In particular, the case corresponding to ionization of one of the electrons while the other one remains in a bound state of the residual target is analysed. The influence of the dynamic correlation between electrons, which is included in the model through the simultaneous time coupling of their evolutions during the collision, is analysed for the proton–helium system under different physical conditions.

Ionization of water molecules by ion beams. On the relevance of dynamic screening and the influence of the description of the initial state

Single ionization from water molecules by impact of protons, alpha particles and C6 + ions is studied. The post- and prior-versions of the continuum distorted wave-eikonal initial state (CDW-EIS) model within an independent electron approximation are employed to compute double differential cross sections. To avoid the complexity of using numerical molecular continuum states in the cross-section calculations, effective Coulombic continuum wavefunctions are employed. However, this may lead to the appearance of post–prior discrepancies and this fact is examined in detail. Moreover, the influence of the dynamic screening on this behaviour is studied. In addition, the contribution of different molecular orbitals to the angular spectrum is analysed for several ejection electron energies. Finally, the sensitivity of CDW-EIS calculations to the representation of the initial bound molecular orbitals is investigated.