The Exploitation of Low-Energy Electrons in Cancer Treatment

Novel Approach for Predicting Vertical Electron Attachment Energies in Bulk-Solvated Molecules

When low-energy electrons interact with molecules, they can give rise to transient anion states commonly known as resonances. These states are formed through vertical electron attachment processes and have the potential to induce various forms of DNA lesions, including base damage, single- and double-strand breaks, cross-links, and clustered lesions that are challenging to repair. So far, most experimental and theoretical studies have investigated the formation of resonances of (bio)molecules in the gas phase or in microsolvated environments. Since cellular environments are mainly composed of water molecules, it is crucial to understand how bulk water affects the resonances of (bio)molecules. Given the existing gap in studies on resonances of bulk-solvated molecules, we propose a novel theoretical-computational approach to address this void. Our approach combines the multibasis-set (time-dependent-)density functional theory and self-consistent sequential quantum mechanics/molecular mechanics polarizable electrostatic embedding methods. We apply this combined methodology to predict the vertical electron attachment energies of 1-methyl-5-nitroimidazole (1M5NI), a well-known radiosensitizer model, in bulk water. In addition, we analyze the rapid mutual polarization between the resonances (both shape- and core-excited) of 1M5NI and the surrounding bulk water environment. For comparison, we also studied the isolated and microsolvated 1M5NI. Overall, while the polarization of the environment is clearly sensitive to the solute charge, causing a significant impact on the vertical electron affinity and consequently on the attachment electron energies, it does not have a significant impact on the excitation energies of the anion.

Unraveling the Complexity of DNA Radiation Damage Using DNA Nanotechnology

Radiation cancer therapies use different ionizing radiation qualities that damage DNA molecules in tumor cells by a yet not completely understood plethora of mechanisms and processes. While the direct action of the radiation is significant, the byproducts of the water radiolysis, mainly secondary low-energy electrons (LEEs, <20 eV) and reactive oxygen species (ROS), can also efficiently cause DNA damage, in terms of DNA strand breakage or DNA interstrand cross-linking. As a result, these types of DNA damage evolve into mutations hindering DNA replication, leading to cancer cell death. Concomitant chemo-radiotherapy explores the addition of radiosensitizing therapeutics commonly targeting DNA, such as platinum derivatives and halogenated nucleosides, to enhance the harmful effects of ionizing radiation on the DNA molecule. Further complicating the landscape of DNA damage are secondary structures such as G-quadruplexes occurring in telomeric DNA. These structures protect DNA from radiation damage, rendering them as promising targets for new and more selective cancer radiation treatments, rather than targeting linear DNA. However, despite extensive research, there is no single paradigm approach to understanding the mysterious way in which ionizing radiation causes DNA damage. This is due to the multidisciplinary nature of the field of research, which deals with multiple levels of biological organization, from the molecular building blocks of life toward cells and organisms, as well as with complex multiscale radiation-induced effects. Also, intrinsic DNA features, such as DNA topology and specific oligonucleotide sequences, strongly influence its response to damage from ionizing radiation. In this Account, we present our studies focused on the absolute quantification of photon- and low-energy electron-induced DNA damage in strategically selected target DNA sequences. Our methodology involves using DNA origami nanostructures, specifically the Rothemund triangle, as a platform to expose DNA sequences to either low-energy electrons or vacuum-ultraviolet (VUV, <15 eV) photons and subsequent atomic force microscopy (AFM) analysis. Through this approach, the effects of the DNA sequence, incorporation of halogenated radiosensitizers, DNA topology, and the radiation quality on radiation-induced DNA strand breakage have been systematically assessed and correlated with fundamental photon- and electron-driven mechanisms underlying DNA radiation damage. At lower energies, these mechanisms include dissociative electron attachment (DEA), where electrons attach to DNA molecules causing strand breaks, and dissociative photoexcitation of DNA. Additionally, further dissociative processes such as photoionization and electron impact contribute to the complex cascade of DNA damage events induced by ionizing radiation. We expect that emerging DNA origami-based approaches will lead to a paradigm shift in research fields associated with DNA damage and suggest future directions, which can foster the development of technological applications in nanomedicine, e.g., optimized cancer treatments or the molecular design of optimized radiosensitizing therapeutics.

Interaction of low-energy electrons with radiosensitizers

We provide an experimentalist's perspective on the present state-of-the-art in the studies of low-energy electron interactions with common radiosensitizers, including compounds used in combined chemo-radiation therapy and their model systems. Low-energy electrons are important secondary species formed during the interaction of ionizing radiation with matter. Their role in the radiation chemistry of living organisms has become an important topic for more than 20 years. With the increasing number of works and reviews in the field, we would like to focus here on a very narrow area of compounds that have been shown to have radio-sensitizing properties on the one hand, and high reactivity towards low-energy electrons on the other hand. Gas phase experiments studying electron attachment to isolated molecules and environmental effects on reaction dynamics are reviewed for modified DNA components, nitroimidazoles, and organometallics. In the end, we provide a perspective on the future directions that may be important for transferring the fundamental knowledge about the processes induced by low-energy electrons into practice in the field of rational design of agents for concomitant chemo-radiation therapy.

The Effects of Particle LET and Fluence on the Complexity and Frequency of Clustered DNA Damage

Motivation

Clustered DNA-lesions are predominantly induced by ionizing radiation, particularly by high-LET particles, and considered as lethal damage. Quantification of this specific type of damage as a function of radiation parameters such as LET, dose rate, dose, and particle type can be informative for the prediction of biological outcome in radiobiological studies. This study investigated the induction and complexity of clustered DNA damage for three different types of particles at an LET range of 0.5-250 keV/μm.

Methods

Nanometric volumes (36.0 nm3) of 15 base-pair DNA with its hydration shell was modeled. Electron, proton, and alpha particles at various energies were simulated to irradiate the nanometric volumes. The number of ionization events, low-energy electron spectra, and chemical yields for the formation of °OH, H°, e aq - , and H2O2 were calculated for each particle as a function of LET. Single- and double-strand breaks (SSB and DSB), base release, and clustered DNA-lesions were computed from the Monte-Carlo based quantification of the reactive species and measured yields of the species responsible for the DNA lesion formation.

Results

The total amount of DNA damage depends on particle type and LET. The number of ionization events underestimates the quantity of DNA damage at LETs higher than 10 keV/μm. Minimum LETs of 9.4 and 11.5 keV/μm are required to induce clustered damage by a single track of proton and alpha particles, respectively. For a given radiation dose, an increase in LET reduces the number of particle tracks, leading to more complex clustered DNA damage, but a smaller number of separated clustered damage sites.

Conclusions

The dependency of the number and the complexity of clustered DNA damage on LET and fluence suggests that the quantification of this damage can be a useful method for the estimation of the biological effectiveness of radiation. These results also suggest that medium-LET particles are more appropriate for the treatment of bulk targets, whereas high-LET particles can be more effective for small targets.

Role of Low-Energy (<20 eV) Secondary Electrons in the Extraterrestrial Synthesis of Prebiotic Molecules

We demonstrate for the first time that Galactic cosmic rays with energies as high as ∼1010 eV can trigger a cascade of low-energy (<20 eV) secondary electrons that could be a significant contributor to the interstellar synthesis of prebiotic molecules whose delivery by comets, meteorites, and interplanetary dust particles may have kick-started life on Earth. For the energetic processing of interstellar ice mantles inside dark, dense molecular clouds, we explore the relative importance of low-energy (<20 eV) secondary electrons-agents of radiation chemistry-and low-energy (<10 eV), nonionizing photons-instigators of photochemistry. Our calculations indicate fluxes of ∼102 electrons cm-2 s-1 for low-energy secondary electrons produced within interstellar ices due to attenuated Galactic cosmic-ray protons. Consequently, in certain star-forming regions where internal high-energy radiation sources produce ionization rates that are observed to be a thousand times greater than the typical interstellar Galactic ionization rate, the flux of low-energy secondary electrons should far exceed that of nonionizing photons. Because reaction cross sections can be several orders of magnitude larger for electrons than for photons, even in the absence of such enhancement, our calculations indicate that secondary low-energy (<20 eV) electrons are at least as significant as low-energy (<10 eV) nonionizing photons in the interstellar synthesis of prebiotic molecules. Most importantly, our results demonstrate the pressing need for explicitly incorporating low-energy electrons in current and future astrochemical simulations of cosmic ices. Such models are critically important for interpreting James Webb Space Telescope infrared measurements, which are currently being used to probe the origins of life by studying complex organic molecules found in ices near star-forming regions.

Fragmentation of the DNA Lesion 8-oxo-Guanine by Low-Energy Electrons

8-oxo-Guanine is a mutagenic lesion produced by reactions involving reactive oxygen species and guanine in DNA. Its production induces mispairing between the canonical nucleobases during DNA replication such that various types of cancers are associated with the DNA lesion. Since radiation therapy is used in some cases, the interaction of low-energy electrons with 8-oxo-guanine can in turn produce other reactive species, which in principle could have either a detrimental or protective effect on the organism. Motivated by these facts, we report a comparative experimental study of electron-induced fragmentation of guanine and 8-oxo-guanine, along with a theoretical study of the π* shape resonances and bound anion states, which may trigger those dissociation reactions. The electron-induced fragmentation of 8-oxo-guanine is remarkably distinct from the native form. More complex reactions were observed for the oxidized species, which may produce several anion fragments at very low energies (∼0 eV). The dehydrogenated parent anion, which is already a minor fragment in guanine, was completely suppressed in 8-oxo-guanine. The calculated thermodynamical thresholds also suggest that NH2 elimination in guanine, at sub-excitation energies, proceeds via a complex reaction involving rearrangement steps.

Low-Energy Electron Generation for Biomolecular Damage Inquiry: Instrumentation and Methods

Technological advancement has produced a variety of instruments and methods to generate electron beams that have greatly assisted in the extensive theoretical and experimental efforts devoted to investigating the effect of secondary electrons with energies approximately less than 100 eV, which are referred as low-energy electrons (LEEs). In the past two decades, LEE studies have focused on biomolecular systems, which mainly consist of DNA and proteins and their constituents as primary cellular targets of ionizing radiation. These studies have revealed that compared to other reactive species produced by high-energy radiation, LEEs have distinctive pathways and considerable efficiency in inducing lethal DNA lesions. The present work aims to briefly discuss the current state of LEE production technology and to motivate further studies and improvements of LEE generation techniques in relation to biological electron-driven processes associated with such medical applications as radiation therapy and cancer treatment.

Low-Energy Electron Damage to Plasmid DNA in Thin Films: Dependence on Substrates, Surface Density, Charging, Environment, and Uniformity

The interaction of low-energy electrons (LEEs) with DNA plays a significant role in the mechanisms leading to biological damage induced by ionizing radiation, particularly in radiotherapy, and its sensitization by chemotherapeutic drugs and nanoparticles. Plasmids constitute the form of DNA found in mitochondria and appear as a suitable model of genomic DNA. In a search for the best LEE targets, damage was induced to plasmids, in thin films in vacuum, by 6, 10, and 100 eV electrons under single collision conditions. The yields of single- and double-strand breaks, other cluster damage, isolated base lesions, and crosslinks were measured by electrophoresis and enzyme treatment. The films were deposited on oriented graphite or polycrystalline tantalum, with or without DNA autoassembly via diaminopropane (Dap) intercalation. Yields were correlated with the influence of vacuum, film uniformity, surface density, substrates, and the DNA environment. Aided by surface potential measurements and scanning electron microscopy and atomic force microscopy images, the lyophilized Dap-DNA films were found to be the most practical high-quality targets. These studies pave the way to the fabrication of LEE target-films composed of plasmids intercalated with biomolecules that could mimic the cellular environment; for example, as a first step, by replacing Dap with an amino acid.

Dosimetry assessment of theranostic Auger-emitting radionuclides in a micron-sized multicellular cluster model: A Monte Carlo study using Geant4-DNA simulations

The present work is aimed at improving the multicellular dosimetry of several Auger radionuclides of interest for targeted cancer therapy, including ^99mTc, ¹¹¹In, ¹²³I, ¹²⁵I, and ²⁰¹Tl. For this purpose, using the Geant4-DNA Monte Carlo code, a cluster of 13 similar spherical cells with a hexagonal packed arrangement was modeled, and the mean absorbed doses per unit cumulated activity (S-values) were calculated by considering two target←source configurations, cell←cell and nucleus←nucleus. The obtained ratios of cross-dose to self-dose S-value in terms of the distance between the source and target regions were evaluated and also compared to those estimated by the Medical Internal Radiation Dose (MIRD) method. Besides, the contribution of the Coster-Kronig, Auger and internal conversion electrons to the S-values was provided for each radionuclide. According to the results, it can be concluded that in contrast to self-absorption, the cross-absorption due to the Auger-emitters has not a significant role in the total energy deposition within a cell in the cluster.

Low-Energy (5-20 eV) Electron-Induced Single and Double Strand Breaks in Well-Defined DNA Sequences

Ionizing radiation is used in cancer radiation therapy to effectively damage the DNA of tumors. The main damage is due to generation of highly reactive secondary species such as low-energy electrons (LEEs). The accurate quantification of DNA radiation damage of well-defined DNA target sequences in terms of absolute cross sections for LEE-induced DNA strand breaks is possible by the DNA origami technique; however, to date, it is possible only for DNA single strands. In the present work DNA double strand breaks in the DNA sequence 5'-d(CAC)₄/5'-d(GTG)₄ are compared with DNA single strand breaks in the oligonucleotides 5'-d(CAC)₄ and 5'-d(GTG)₄ upon irradiation with LEEs in the energy range from 5 to 20 eV. A maximum of strand break cross section was found around 7 and 10 eV independent of the DNA sequence, indicating that dissociative electron attachment is the underlying mechanism of strand breakage and confirming previous studies using plasmid DNA.

Dissociative electron attachment to 5-bromo-uracil: non-adiabatic dynamics on complex-valued potential energy surfaces

Electron induced dissociation reactions are relevant to many fields, ranging from prebiotic chemistry to cancer treatments. However, the simulation of dissociation electron attachment (DEA) dynamics is very challenging because the auto-ionization widths of the transient negative ions must be accounted for. We propose an adaptation of the ab initio multiple spawning (AIMS) method for complex-valued potential energy surfaces, along the lines of recent developments based on surface hopping dynamics. Our approach combines models for the energy dependence of the auto-ionization widths, obtained from scattering calculations, with survival probabilities computed for the trajectory basis functions employed in the AIMS dynamics. The method is applied to simulate the DEA dynamics of 5-bromo-uracil in full dimensionality, i.e., taking all the vibrational modes into consideration. The propagation starts on the resonance state and describes the formation of Br^- anions mediated by non-adiabatic couplings. The potential energies, gradients and non-adiabatic couplings were computed with the fractional-occupancy molecular orbital complete-active-space configuration-interaction method, and the calculated DEA cross section are consistent with the observed DEA intensities.

Nucleic Acids under Stress: Understanding and Simulating Nucleobase Fragmentation Pathways

The effects of radiations on nucleic acids and their constituents is widely studied across several research fields using different experimental and theoretical protocols. While a large number of studies were performed in this context, many fundamental physical and chemical effects are still being investigated, particularly involving the effect of the biological environment. As an example, the interpretation of experimental nucleic acid bases mass spectra, and hence inferring their reactivity in complex environment still poses great challenge. This Minireview summarizes recent theoretical advancements aiming to predict and interpret the reactivity of nucleic acid bases. We focus not only on the understanding of the inherent fragmentation pathways of isolated nucleobases but also on the modeling of a realistic nano-environments highlighting the importance of molecular dynamics simulations and the non-innocent role of the environment and also the possibility to open novel fragmentation pathways.

Design, Synthesis, and Cytotoxicity Assessment of [64Cu]Cu-NOTA-Terpyridine Platinum Conjugate: A Novel Chemoradiotherapeutic Agent with Flexible Linker

Maximum benefits of chemoradiation therapy with platinum-based compounds are expected if the radiation and the drug are localized simultaneously in cancer cells. To optimize this concomitant effect, we developed the novel chemoradiotherapeutic agent [⁶⁴Cu]Cu-NOTA-C3-TP by conjugating, via a short flexible alkyl chain spacer (C3), a terpyridine platinum (TP) moiety to a NOTA chelator complexed with copper-64 (⁶⁴Cu). The decay of ⁶⁴Cu produces numerous low-energy electrons, enabling the ⁶⁴Cu-conjugate to deliver radiation energy close to TP, which intercalates into G-quadruplex DNA. Accordingly, the in vitro internalization kinetic and the cytotoxic activity of [⁶⁴Cu]Cu-NOTA-C3-TP and its derivatives were investigated with colorectal cancer (HCT116) and normal human fibroblast (GM05757) cells. Radiolabeling by ⁶⁴Cu results in a >55,000-fold increase of cytotoxic potential relative to [^NatCu]Cu-NOTA-C3-TP at 72 h post administration, indicating a large additive effect between ⁶⁴Cu and the TP drug. The internalization and nucleus accumulation of [⁶⁴Cu]Cu-NOTA-C3-TP in the HCT116 cells were, respectively, 3.1 and 6.0 times higher than that for GM05757 normal human fibroblasts, which is supportive of the higher efficiency of the [⁶⁴Cu]Cu-NOTA-C3-TP for HCT116 cancer cells. This work presents the first proof-of-concept study showing the potential use of the [⁶⁴Cu]Cu-NOTA-C3-TP conjugate as a targeted chemoradiotherapeutic agent to treat colorectal cancer.

Low-Energy Electron Damage to Condensed-Phase DNA and Its Constituents

The complex physical and chemical reactions between the large number of low-energy (0-30 eV) electrons (LEEs) released by high energy radiation interacting with genetic material can lead to the formation of various DNA lesions such as crosslinks, single strand breaks, base modifications, and cleavage, as well as double strand breaks and other cluster damages. When crosslinks and cluster damages cannot be repaired by the cell, they can cause genetic loss of information, mutations, apoptosis, and promote genomic instability. Through the efforts of many research groups in the past two decades, the study of the interaction between LEEs and DNA under different experimental conditions has unveiled some of the main mechanisms responsible for these damages. In the present review, we focus on experimental investigations in the condensed phase that range from fundamental DNA constituents to oligonucleotides, synthetic duplex DNA, and bacterial (i.e., plasmid) DNA. These targets were irradiated either with LEEs from a monoenergetic-electron or photoelectron source, as sub-monolayer, monolayer, or multilayer films and within clusters or water solutions. Each type of experiment is briefly described, and the observed DNA damages are reported, along with the proposed mechanisms. Defining the role of LEEs within the sequence of events leading to radiobiological lesions contributes to our understanding of the action of radiation on living organisms, over a wide range of initial radiation energies. Applications of the interaction of LEEs with DNA to radiotherapy are briefly summarized.

Early Events in Radiobiology: Isolated and Cluster DNA Damage Induced by Initial Cations and Nonionizing Secondary Electrons

Radiobiological damage is principally triggered by an initial cation and a secondary electron (SE). We address the fundamental questions: What lesions are first produced in DNA by this cation or nonionizing SE? What are their relative contributions to isolated and potentially lethal cluster lesions? Five monolayer films of dry plasmid DNA deposited on graphite or tantalum substrates are bombarded by 0.1-100 eV electrons in a vacuum. From measurements of the current transmitted through the films, 3.5 and 4.5 cations per incident 60 and 100 eV electrons, respectively, are estimated to be produced and stabilized within DNA. Damage analysis at 6, 10, 20, 30, 60, and 100 eV indicates that essentially all lesions, but preferentially cluster damages, are produced by non-ionizing or weakly ionizing electrons of energies below 12 eV. Most of these lesions are induced within femtosecond times, via transient anions and electron transfer within DNA, with little contributions from the numerous cations.

Advancements in the use of Auger electrons in science and medicine during the period 2015-2019

Auger electrons can be highly radiotoxic when they are used to irradiate specific molecular sites. This has spurred basic science investigations of their radiobiological effects and clinical investigations of their potential for therapy. Focused symposia on the biophysical aspects of Auger processes have been held quadrennially. This 9th International Symposium on Physical, Molecular, Cellular, and Medical Aspects of Auger Processes at Oxford University brought together scientists from many different fields to review past findings, discuss the latest studies, and plot the future work to be done. This review article examines the research in this field that was published during the years 2015-2019 which corresponds to the period since the last meeting in Japan. In addition, this article points to future work yet to be done. There have been a plethora of advancements in our understanding of Auger processes. These advancements range from basic atomic and molecular physics to new ways to implement Auger electron emitters in radiopharmaceutical therapy. The highly localized doses of radiation that are deposited within a 10 nm of the decay site make them precision tools for discovery across the physical, chemical, biological, and medical sciences.

DNA Nucleobase under Ionizing Radiation: Unexpected Proton Transfer by Thymine Cation in Water Nanodroplets

The effect of ionizing radiation on DNA constituents is a widely studied fundamental process using experimental and computational techniques. In particular, radiation effects on nucleobases are usually tackled by mass spectrometry in which the nucleobase is embedded in a water nanodroplet. Here, we present a multiscale theoretical study revealing the effects and the dynamics of water droplets towards neutral and ionized thymine. In particular, by using both hybrid quantum mechanics/molecular mechanics and full ab initio molecular dynamics, we reveal an unexpected proton transfer from thymine cation to a nearby water molecule. This leads to the formation of a neutral radical thymine and a Zundel structure, while the hydrated proton localizes at the interface between the deprotonated thymine and the water droplet. This observation opens entirely novel perspectives concerning the reactivity and further fragmentation of ionized nucleobases.

Low energy (6-18 eV) electron scattering from condensed thymidine (dT) III: absolute electronic excitation cross sections

Absolute cross sections (CSs) for electronic excitation by low-energy electron (LEE) scattering, from condensed thymidine (dT) in the 6-18 eV incident energy range, were measured by high-resolution electron energy loss spectroscopy (HREELS). Various electron energy loss (EEL) spectra were acquired using 1 ML of dT condensed on a multilayer film of Ar held at about 20 K under ultra-high vacuum (∼1 × 10-11 Torr). dT is one of the most complex DNA constituents to be studied by HREELS and these spectra provide the first LEE energy-loss data for electronic excitation of a nucleoside. CSs for transitions to the states 13A', 13A'', 23A', 21A', 33A', 23A'', 43A', 33A'', 53A' and 51A' of dT were extracted from the EEL spectra. These states correlate to those previously measured for the thymine moiety. Two broad resonances are observed in the energy dependence of the CSs at around 8 and 10 eV; these energies are close to those found in earlier gas- and solid-phase studies on the interaction of LEEs with dT, thymine and related molecules. A quantitative comparison between the electronic CSs of dT and those of thymine and tetrahydrofuran indicates that no variation is induced in the electronic CSs of thymine upon chemically binding to a deoxyribose group.

Synergistic Radiosensitization by Gold Nanoparticles and the Histone Deacetylase Inhibitor SAHA in 2D and 3D Cancer Cell Cultures

Radiosensitizing agents are capable of augmenting the damage of ionizing radiation preferentially on cancer cells, thereby increasing the potency and the specificity of radiotherapy. Metal-based nanoparticles have recently gathered ground in radio-enhancement applications, owing to their exceptional competence in amplifying the cell-killing effects of irradiation. Our aim was to examine the radiosensitizing performance of gold nanoparticles (AuNPs) and the chromatin-modifying histone deacetylase inhibitor suberoylanilide hydroxamic acid (SAHA) alone and in combination. We observed that the colony-forming capability of cancer cells decreased significantly and the DNA damage, detected by γH2AX immunostaining, was substantially greater after combinational treatments than upon individual drug exposures followed by irradiation. Synergistic radiosensitizing effects of AuNPs and SAHA were proven on various cell lines, including radioresistant A549 and DU-145 cancer cells. 3D cultures often manifest radio- and drug-resistance, nevertheless, AuNPs in combination with SAHA could effectively enhance the potency of irradiation as the number of viable cells decreased significantly when spheroids received AuNP + SAHA prior to radiotherapy. Our results imply that a relaxed chromatin structure induced by SAHA renders the DNA of cancerous cells more susceptible to the damaging effects of irradiation-triggered, AuNP-released reactive electrons. This feature of AuNPs should be exploited in multimodal treatment approaches.

Low energy (1-19 eV) electron scattering from condensed thymidine (dT) I: absolute vibrational excitation cross sections

Absolute cross sections (CSs) for vibrational excitation by electrons of energy between 1-19 eV scattering from condensed thymidine (dT) were measured by means of high-resolution electron energy loss spectroscopy (HREELS). The CSs were extracted from electron energy loss spectra of dT condensed on multilayers film of Ar held at about 20 K under ultra-high vacuum (∼1 × 10-11 Torr). dT is one of the most complex molecules to be studied in condensed phase by HREELS. The magnitudes of the vibrational CSs lie within the 10-17 cm2 range. Structures observed in the energy dependence of the vibrational CSs under 3 eV and around 4 eV were compared with previous results of gas- and solid-phase studies on dT and related molecules (e.g., thymine and tetrahydrofuran). These structures were attributed to the formation of shape resonances.

Clustered DNA Damages induced by 0.5 to 30 eV Electrons

Low-energy electrons (LEEs) of energies ≤30 eV are generated in large quantities by ionizing radiation. These electrons can damage DNA; particularly, they can induce the more detrimental clustered lesions in cells. This type of lesions, which are responsible for a large portion of the genotoxic stress generated by ionizing radiation, is described in the Introduction. The reactions initiated by the collisions of 0.5-30 eV electrons with oligonucleotides, duplex DNA, and DNA bound to chemotherapeutic platinum drugs are explained and reviewed in the subsequent sections. The experimental methods of LEE irradiation and DNA damage analysis are described with an emphasis on the detection of cluster lesions, which are considerably enhanced in DNA-Pt-drug complexes. Based on the energy dependence of damage yields and cross-sections, a mechanism responsible for the clustered lesions can be attributed to the capture of a single electron by the electron affinity of an excited state of a base, leading to the formation of transient anions at 6 and 10 eV. The initial capture is followed by electronic excitation of the base and dissociative attachment-at other DNA sites-of the electron reemitted from the temporary base anion. The mechanism is expected to be universal in the cellular environment and plays an important role in the formation of clustered lesions.

Clustered DNA Damage Induced by 2-20 eV Electrons and Transient Anions: General Mechanism and Correlation to Cell Death

The mechanisms of action of low-energy electrons (LEEs) generated in large quantities by ionizing radiation constitute an essential element of our understanding of early events in radiolysis and radiobiology. We present the 2-20 eV electron energy dependence of the yields of base damage (BD), BD-related cross-links (CLs), and non-double-strand break (NDSB) clustered damage induced in DNA. These new yield functions are generated by the impact of LEEs on plasmid DNA films. The damage is analyzed by gel electrophoresis with and without enzyme treatment. Maxima at 5 and 10 eV in BDs and BD-related CLs yield functions, and two others, at 6 and 10 eV, in those of NDSB clustered damage are ascribed to core-excited transient anions that decay into bond-breaking channels. The mechanism causing all types of DNA damages can be attributed to the capture of a single electron by a base followed by multiple different electron transfer pathways.

Absolute cross section for DNA damage induced by low-energy (10 eV) electrons: Experimental refinements and sample characterization by AFM

This work describes multiple experimental improvements for measuring absolute cross sections of DNA damage induced by low-energy electrons in nanometer-thick films in vacuum. Measurements of such cross sections are particularly sensitive to film thickness and uniformity. Using atomic force microscopy in 70% ethanol, we present a novel and effective method to determine plasmid DNA film thickness and uniformity that combines height histograms and force-distance curves. We also investigate film deposition with DNA intercalated with 1,3-diaminopropane (Dap) on tantalum-coated substrates as a convenient and cost-effective alternative to the previously-used graphite substrate. The tantalum substrate permits deposition of films very similar to those formed on graphite. Using these refinements and further optimizations of the experimental procedure, we measure an absolute cross section of (7.4 ± 2.3) × 10^-18 cm² per nucleotide for conformational damage to a 3197 base-pair plasmid, induced by 10 eV electrons, which we believe should be considered as a reference value.

The Physico-Chemical Basis of DNA Radiosensitization: Implications for Cancer Radiation Therapy

High-energy radiation is used in combination with radiosensitizing therapeutics to treat cancer. The most common radiosensitizers are halogenated nucleosides and cisplatin derivatives, and recently also metal nanoparticles have been suggested as potential radiosensitizing agents. The radiosensitizing action of these compounds can at least partly be ascribed to an enhanced reactivity towards secondary low-energy electrons generated along the radiation track of the high-energy primary radiation, or to an additional emission of secondary reactive electrons close to the tumor tissue. This is referred to as physico-chemical radiosensitization. In this Concept article we present current experimental methods used to study fundamental processes of physico-chemical radiosensitization and discuss the most relevant classes of radiosensitizers. Open questions in the current discussions are identified and future directions outlined, which can lead to optimized treatment protocols or even novel therapeutic concepts.

Electron Attachment to Microhydrated Deoxycytidine Monophosphate

DNA constituents are effectively decomposed via dissociative electron attachment (DEA). However, the DEA contribution to radiation damage in living tissues is a subject of ongoing discussion. We address an essential question, how aqueous environment influences the DEA to DNA. In particular, we report experimental fragmentation patterns for DEA to microhydrated 2-deoxycytidine 5-monophosphate (dCMP). Isolated dCMP was previously set as a model to describe mechanisms of DNA-strand breaks induced by secondary electrons and decomposes primarily by dissociation of the C-O phosphoester bond. We show that hydrated molecules decompose via dissociation of the C-N glycosidic bond followed by dissociation of the P-O bond. This significant change of the proposed mechanism can be interpreted by a reactive role of water in the postattachment dynamics. Comparison of the fragmentation with previous macroscopic irradiation studies suggests that the actual contribution of DEA to DNA radiation damage in living tissue is rather small.