Measurements of G values for DNA damage induced by low-energy electrons

Role of Overlap between the Discrete State and Pseudocontinuum States in Stabilization Calculations of Metastable States

In a diabatic picture metastable states subject to decay by electron detachment can be viewed as arising from the coupling between a discrete state and a continuum. In treating such states with bound-state quantum chemical methods, the continuum is discretized. In this study, we elucidate the role of overlap in this interaction in the application of the stabilization method to temporary anion states. This is accomplished by use of a minimalist stabilization calculation on the lowest energy l=2 (D) resonance of the finite spherical well potential using two basis functions, one describing the diabatic discrete state and the other a diabatic discretized continuum state. We show that even such a simple treatment predicts a complex resonance energy in good agreement with the exact result. If the energy of the discrete state is assumed to be constant, which is tantamount to orthogonalizing the discretized continuum state to the discrete state, it is demonstrated that the square of the off-diagonal coupling has a maximum close to the crossing point of the orthogonalized diabatic curves and that the curvature in the coupling is responsible for the complex stationary point associated with the resonance. Moreover, this curvature is a consequence of the overlap between the two diabatic states.

Base Release and Modification in Solid-Phase DNA Exposed to Low-Energy Electrons

Ionization generates a large number of secondary low-energy electrons (LEEs) with a most probable energy of approximately 10 eV, which can break DNA bonds by dissociative electron attachment (DEA) and lead to DNA damage. In this study, we investigated radiation damage to dry DNA induced by X rays (1.5 keV) alone on a glass substrate or X rays combined with extra LEEs (average energy of 5.8 eV) emitted from a tantalum (Ta) substrate under an atmosphere of N₂ and standard ambient conditions of temperature and pressure. The targets included calf-thymus DNA and double-stranded synthetic oligonucleotides. We developed analytical methods to measure the release of non-modified DNA bases from DNA and the formation of several base modifications by LC-MS/MS with isotopic dilution for precise quantification. The results show that the yield of non-modified bases as well as base modifications increase by 20-30% when DNA is deposited on a Ta substrate compared to that on a glass substrate. The order of base release (Gua > Ade > Thy ∼ Cyt) agrees well with several theoretical studies indicating that Gua is the most susceptible site toward sugar-phosphate cleavage. The formation of DNA damage by LEEs is explained by DEA leading to the release of non-modified bases involving the initial cleavage of N1-C1', C3'-O3' or C5'-O5' bonds. The yield of base modifications was lower than the release of non-modified bases. The main LEE-induced base modifications include 5,6-dihydrothymine (5,6-dHT), 5,6-dihydrouracil (5-dHU), 5-hydroxymethyluracil (5-HmU) and 5-formyluracil (5-ForU). The formation of base modifications by LEEs can be explained by DEA and cleavage of the C-H bond of the methyl group of Thy (giving 5-HmU and 5-ForU) and by secondary reactions of H atoms and hydride anions that are generated by primary LEE reactions followed by subsequent reaction with Cyt and Thy (giving 5,6-dHU and 5,6-dHT).

Ab initio calculation of the cross sections for electron impact vibrational excitation of CO via the (2)Π shape resonance

The stabilization method is used to calculate the complex potential energy curve of the (2)Π state of CO(-) as a function of bond length, with the refinement that separate potentials are determined for p-wave and d-wave attachment and detachment of the excess electron. Using the resulting complex potentials, absolute vibrational excitation cross sections are calculated as a function of electron energy and scattering angle. The calculated cross sections agree well with experiment.

Loss of cellular transformation efficiency induced by DNA irradiation with low-energy (10 eV) electrons

Low energy electrons (LEEs) of energies less than 20 eV are generated in large quantities by ionizing radiation in biological matter. While LEEs are known to induce single (SSBs) and double strand breaks (DSBs) in DNA, their ability to inactivate cells by inducing nonreparable lethal damage has not yet been demonstrated. Here we observe the effect of LEEs on the functionality of DNA, by measuring the efficiency of transforming Escherichia coli with a [pGEM-3Zf (-)] plasmid irradiated with 10 eV electrons. Highly ordered DNA films were prepared on pyrolitic graphite by molecular self-assembly using 1,3-diaminopropane ions (Dap(2+)). The uniformity of these films permits the inactivation of approximately 50% of the plasmids compared to <10% using previous methods, which is sufficient for the subsequent determination of their functionality. Upon LEE irradiation, the fraction of functional plasmids decreased exponentially with increasing electron fluence, while LEE-induced isolated base damage, frank DSB, and non DSB-cluster damage increased linearly with fluence. While DSBs can be toxic, their levels were too low to explain the loss of plasmid functionality observed upon LEE irradiation. Similarly, non-DSB cluster damage, revealed by transforming cluster damage into DSBs by digestion with repair enzymes, also occurred relatively infrequently. The exact nature of the lethal damage remains unknown, but it is probably a form of compact cluster damage in which the lesions are too close to be revealed by purified repair enzymes. In addition, this damage is either not repaired or is misrepaired by E. coli, since it results in plasmid inactivation, when they contain an average of three lesions. Comparison with previous results from a similar experiment performed with γ-irradiated plasmids indicates that the type of clustered DNA lesions, created directly on cellular DNA by LEEs, may be more difficult to repair than those produced by other species from radiolysis.

Side-by-side comparison of DNA damage induced by low-energy electrons and high-energy photons with solid TpTpT trinucleotide

The genotoxic effects of high-energy ionizing radiation have been largely attributed to the ionization of H2O leading to hydroxyl radicals and the ionization of DNA leading mostly to damage through base radical cations. However, the contribution of low-energy electrons (LEEs; ≤ 10 eV), which involves subionization events, has been considered to be less important than that of hydroxyl radicals and base radical cations. Here, we compare the ability of LEEs and high-energy X-ray photons to induce DNA damage using dried thin films of TpTpT trinucleotide as a simple and representative model for DNA damage. The main radiation-induced damage of TpTpT as measured by high-performance liquid chromatography (HPLC) with UV detection and HPLC coupled to tandem mass spectrometry analyses included thymine release (-Thy), strand breaks (pT, Tp, pTpT, TpTp, and TpT), and the formation of base modifications [5,6-dihydrothymine (5,6-dhT), 5-hydroxymethyluracil (5-hmU), and 5-formyluracil (5-fU)]. The global profile of products was very similar for both types of radiation indicating converging pathways of formation. The percent damage of thymine release, fragmentation, and base modification was 20, 19, and 61 for high-energy X-rays, respectively, compared to 35, 13, and 51 for LEEs (10 eV). Base release was significantly lower for X-rays. In both cases, phosphodiester bond cleavage gave mononucleotides (pT and Tp) and dinucleotides (pTpT and TpTp) containing a terminal phosphate as the major fragments. For base modifications, the ratio of reductive (5,6-dhT) to oxidative products (5-hmU plus 5-fU) was 0.9 for high-energy X-rays compared to 1.7 for LEEs. These results indicate that LEEs give a similar profile of products compared to ionizing radiation.

The Role of Humidity and Oxygen Level on Damage to DNA Induced by Soft X-rays and Low-Energy Electrons

Single- and double-strand breaks induced by soft X-rays (1.5 keV) and photo-emitted LEEs (0-30 eV) were measured in dry and humid thin films of plasmid DNA irradiated under different controllable levels of oxygen at standard ambient temperature and pressure (SATP). G values derived from these experiments shows that the presence of H2O and changing the atmosphere from N2 to O2, while keeping all other experimental parameters constant, increases the formation of DSBs by factors of 4.5 and 11.8 for X-rays and LEEs, respectively. Under an oxygenated environment in humid DNA films, the additional LEE-induced damage resulting from the combination of water and oxygen exhibits a supper-additive effect, which leads to the formation of DSBs with a G value almost 7 times higher than that obtained by X-ray photons. These results indicate that O2, H2O and LEEs effectively contribute synergistically to enhance the formation of DSBs.

Radiation Damage to DNA: The Indirect Effect of Low Energy Electrons

We report the effect of DNA hydration level on damage yields induced by soft X-rays and photo-emitted low energy electrons (LEEs) in thin films of plasmid DNA irradiated in N₂ at atmospheric pressure under different humidity levels. Contrary to a dilute solution of DNA, the number of H₂O molecules per nucleotide (Γ) in these films can be varied from Γ=2.5 to ~33, where Γ≤20 corresponds to layers of hydration and Γ=33 to an additional bulk-like water layer. Our results indicate that DNA damage induced by LEEs does not increase significantly until the second hydration shell is formed. However, this damage increases dramatically as DNA coverage approaches bulk-like hydration conditions. A number of phenomena are invoked to account for these behaviors including: dissociative electron transfer from water-interface electron traps to DNA bases, quenching of dissociative electron attachment to DNA and quenching of dissociative electronically excited states of H₂O in contact with DNA.

Absolute cross section for low-energy-electron damage to condensed macromolecules: a case study of DNA

Cross sections (CSs) for the interaction of low-energy electrons (LEE) with condensed macromolecules are essential parameters for accurate modeling of radiation-induced molecular decomposition and chemical synthesis. Electron irradiation of dry nanometer-scale macromolecular solid films has often been employed to measure CSs and other quantitative parameters for LEE interactions. Since such films have thicknesses comparable with electron thermalization distances, energy deposition varies throughout the film. Moreover, charge accumulation occurring inside the films shields a proportion of the macromolecules from electron irradiation. Such effects complicate the quantitative comparison of the CSs obtained in films of different thicknesses and limit the applicability of such measurements. Here, we develop a simple mathematical model, termed the molecular survival model, that employs a CS for a particular damage process together with an attenuation length related to the total CS, to investigate how a measured CS might be expected to vary with experimental conditions. As a case study, we measure the absolute CS for the formation of DNA strand breaks (SBs) by electron irradiation at 10 and 100 eV of lyophilized plasmid DNA films with thicknesses between 10 and 30 nm. The measurements are shown to depend strongly on the thickness and charging condition of the nanometer-scale films. Such behaviors are in accord with the model and support its validity. Via this analysis, the CS obtained for SB damage is nearly independent of film thickness and charging effects. In principle, this model can be adapted to provide absolute CSs for electron-induced damage or reactions occurring in other molecular solids across a wider range of experimental conditions.