Fast Ion-Beam Inactivation of Viruses, Where Radiation Track Structure Meets RNA Structural Biology.

Here we show an interplay between the structures present in ionization tracks and nucleocapsid RNA structural biology, using fast ion-beam inactivation of the severe acute respiratory syndrome coronavirus (SARS-CoV) virion as an example. This interplay could be a key factor in predicting dose-inactivation curves for high-energy ion-beam inactivation of virions. We also investigate the adaptation of well-established cross-section data derived from radiation interactions with water to the interactions involving the components of a virion, going beyond the density-scaling approximation developed previously. We conclude that solving one of the grand challenges of structural biology - the determination of RNA tertiary/quaternary structure - is linked to predicting ion-beam inactivation of viruses and that the two problems can be mutually informative. Indeed, our simulations show that fast ion beams have a key role to play in elucidating RNA tertiary/quaternary structure.

Real-Time Electron Solvation Induced by Bursts of Laser-Accelerated Protons in Liquid Water.

Understanding the mechanisms of proton energy deposition in matter and subsequent damage formation is fundamental to radiation science. Here we exploit the picosecond (10^{-12} s) resolution of laser-driven accelerators to track ultrafast solvation dynamics for electrons due to proton radiolysis in liquid water (H_{2}O). Comparing these results with modeling that assumes initial conditions similar to those found in photolysis reveals that solvation time due to protons is extended by >20 ps. Supported by magnetohydrodynamic theory this indicates a highly dynamic phase in the immediate aftermath of the proton interaction that is not accounted for in current models.

Physical Radiation Enhancement Effects Around Clinically Relevant Clusters of Nanoagents in Biological Systems.

Here we show that the determining factor for physical radiation enhancement effects for a clinically realistic cluster of heavy-atom bearing nanoparticles is the total number of heavy atoms packed into the cluster. We do this through a multiscale Monte Carlo approach which permits the consideration of radiation transport through clusters of millions of nanoparticles. The finding is in contrast to that predicted when isolated nanoparticles are considered and is a direct consequence of the Auger electrons playing less of a role for clusters compared to isolate nanoparticles. We further show that this result is agnostic to selection of the subcellular region considered to be sensitive to the effects of radiation, provided the inside the cluster of nanoparticles is not considered to be biologically active.

Protein disulphide isomerase as a target for nanoparticle-mediated sensitisation of cancer cells to radiation.

Radiation resistance and toxicity in normal tissues are limiting factors in the efficacy of radiotherapy. Gold nanoparticles (GNPs) have been shown to be effective at enhancing radiation-induced cell death, and were initially proposed to physically enhance the radiation dose deposited. However, biological responses of GNP radiosensitization based on physical assumptions alone are not predictive of radiosensitisation and therefore there is a fundamental research need to determine biological mechanisms of response to GNPs alone and in combination with ionising radiation. This study aimed to identify novel mechanisms of cancer cell radiosensitisation through the use of GNPs, focusing on their ability to induce cellular oxidative stress and disrupt mitochondrial function. Using N-acetyl-cysteine, we found mitochondrial oxidation to be a key event prior to radiation for the radiosensitisation of cancer cells and suggests the overall cellular effects of GNP radiosensitisation are a result of their interaction with protein disulphide isomerase (PDI). This investigation identifies PDI and mitochondrial oxidation as novel targets for radiosensitisation.

Picosecond metrology of laser-driven proton bursts.

Tracking primary radiation-induced processes in matter requires ultrafast sources and high precision timing. While compact laser-driven ion accelerators are seeding the development of novel high instantaneous flux applications, combining the ultrashort ion and laser pulse durations with their inherent synchronicity to trace the real-time evolution of initial damage events has yet to be realized. Here we report on the absolute measurement of proton bursts as short as 3.5±0.7 ps from laser solid target interactions for this purpose. Our results verify that laser-driven ion acceleration can deliver interaction times over a factor of hundred shorter than those of state-of-the-art accelerators optimized for high instantaneous flux. Furthermore, these observations draw ion interaction physics into the field of ultrafast science, opening the opportunity for quantitative comparison with both numerical modelling and the adjacent fields of ultrafast electron and photon interactions in matter.

Imaging intracellular and systemic in vivo gold nanoparticles to enhance radiotherapy.

Nanoparticles offer alternative options in cancer therapy both as drug delivery carriers and as direct therapeutic agents for cancer cell inactivation. More recently, gold nanoparticles (AuNPs) have emerged as promising radiosensitizers achieving significantly elevated radiation dose enhancement factors when irradiated with both kilo-electron-volt and mega-electron-volt X-rays. Use of AuNPs in radiobiology is now being intensely driven by the desire to achieve precise energy deposition in tumours. As a consequence, there is a growing demand for efficient and simple techniques for detection, imaging and characterization of AuNPs in both biological and tumour samples. Spatially accurate imaging on the nanoscale poses a serious challenge requiring high- or super-resolution imaging techniques. In this mini review, we discuss the challenges in using AuNPs as radiosensitizers as well as various current and novel imaging techniques designed to validate the uptake, distribution and localization in mammalian cells. In our own work, we have used multiphoton excited plasmon resonance imaging to map the AuNP intracellular distribution. The benefits and limitations of this approach will also be discussed in some detail. In some cases, the same "excitation" mechanism as is used in an imaging modality can be harnessed to make it also a part of therapy modality (e.g. phototherapy)-such examples are discussed in passing as extensions to the imaging modality concerned.

Radiosensitising nanoparticles as novel cancer therapeutics--pipe dream or realistic prospect?

The field of high atomic number nanoparticle radiosensitising agents is reviewed. After a brief discussion of the new mode of physicochemical action implied by irradiation of high atomic number nanoparticles embedded in biological systems, a series of exemplars are discussed. Silver-, gadolinium- and gold-based nanoparticles are discussed in order of increasing atomic number with functionalisation strategies being outlined. In vitro and in vivo evidence for radio-enhancement and the mechanisms attributed to the increased biological effect are discussed.

Antiproton induced DNA damage: proton like in flight, carbon-ion like near rest.

Biological validation of new radiotherapy modalities is essential to understand their therapeutic potential. Antiprotons have been proposed for cancer therapy due to enhanced dose deposition provided by antiproton-nucleon annihilation. We assessed cellular DNA damage and relative biological effectiveness (RBE) of a clinically relevant antiproton beam. Despite a modest LET (~19 keV/µm), antiproton spread out Bragg peak (SOBP) irradiation caused significant residual γ-H2AX foci compared to X-ray, proton and antiproton plateau irradiation. RBE of ~1.48 in the SOBP and ~1 in the plateau were measured and used for a qualitative effective dose curve comparison with proton and carbon-ions. Foci in the antiproton SOBP were larger and more structured compared to X-rays, protons and carbon-ions. This is likely due to overlapping particle tracks near the annihilation vertex, creating spatially correlated DNA lesions. No biological effects were observed at 28-42 mm away from the primary beam suggesting minimal risk from long-range secondary particles.

Evaluation of cytotoxicity and radiation enhancement using 1.9 nm gold particles: potential application for cancer therapy.

High atomic number (Z) materials such as gold preferentially absorb kilovoltage x-rays compared to soft tissue and may be used to achieve local dose enhancement in tumours during treatment with ionizing radiation. Gold nanoparticles have been demonstrated as radiation dose enhancing agents in vivo and in vitro. In the present study, we used multiple endpoints to characterize the cellular cytotoxic response of a range of cell lines to 1.9 nm gold particles and measured dose modifying effects following transient exposure at low concentrations. Gold nanoparticles caused significant levels of cell type specific cytotoxicity, apoptosis and increased oxidative stress. When used as dose modifying agents, dose enhancement factors varied between the cell lines investigated with the highest enhancement being 1.9 in AGO-1522B cells at a nanoparticle concentration of 100 microg ml(-1). This study shows exposure to 1.9 nm gold particles to induce a range of cell line specific responses including decreased clonogenic survival, increased apoptosis and induction of DNA damage which may be mediated through the production of reactive oxygen species. This is the first study involving 1.9 nm nanometre sized particles to report multiple cellular responses which impact on the radiation dose modifying effect. The findings highlight the need for extensive characterization of responses to gold nanoparticles when assessing dose enhancing potential in cancer therapy.

Fragmentation and plasmid strand breaks in pure and gold-doped DNA irradiated by beams of fast hydrogen atoms.

The results of an investigation into the damage caused to dry plasmid DNA after irradiation by fast (keV) hydrogen atoms are presented. Agarose gel electrophoresis was used to assess single and double strand break yields as a function of dose in dry DNA samples deposited on a mica substrate. Damage levels were observed to increase with beam energy. Strand break yields demonstrated a considerable dependence on sample structure and the method of sample preparation. Additionally, the effect of high-Z nanoparticles on damage levels was investigated by irradiating DNA samples containing controlled amounts of gold nanoparticles. In contrast to previous (photonic) studies, no enhancement of strand break yields was observed with the particles showing a slight radioprotective effect. A model of DNA damage as a function of dose has been constructed in terms of the probability for the creation of single and double strand breaks, per unit ion flux. This model provides quantitative conclusions about the effects of both gold nanoparticles and the different buffers used in performing the assays and, in addition, infers the proportion of multiply damaged fragments.

Variation of strand break yield for plasmid DNA irradiated with high-Z metal nanoparticles.

Using agarose gel electrophoresis, we measured the effectiveness of high-Z metal particles of different sizes on SSB and DSB yields for plasmid DNA irradiated with 160 kVp X rays. For plasmid samples prepared in Tris-EDTA buffer, gold nanoparticles were shown to increase G'(SSB) typically by a factor of greater than 2 while G'(DSB) increased by a factor of less than 2. Similar dose-modifying effects were also observed using gold microspheres. Addition of 10(-1) M DMSO typically decreased damage yields by a factor of less than 0.5. Plasmid samples prepared in PBS showed significantly different damage yields compared to those prepared in Tris-EDTA (P < 0.001) with G'(SSB) and G'(DSB) increasing by factors of 100 and 48, respectively. Furthermore, addition of gold nanoparticles to samples prepared in PBS decreased G'(SSB) and G'(DSB) by factors of 0.2 and 0.3, respectively. The results show plasmid damage yields to be highly dependent on differences in particle size between the micro- and nanometer scale, atomic number (Z) of the particle, and scavenging capacity of preparation buffers. This study provides further evidence using a plasmid DNA model system for the potential of high-Z metal nanoparticles as local dose-modifying agents.

Isotope shift in the dielectronic recombination of three-electron ANd57+.

Isotope shifts in dielectronic recombination spectra were studied for Li-like (A)Nd(57+) ions with A=142 and A=150. From the displacement of resonance positions energy shifts deltaE(142 150)(2s-2p(1/2))=40.2(3)(6) meV [(stat)(sys)] and deltaE(142 150)(2s-2p(3/2))=42.3(12)(20) meV of 2s-2p(j) transitions were deduced. An evaluation of these values within a full QED treatment yields a change in the mean-square charge radius of (142 150)deltar(2)=-1.36(1)(3) fm(2). The approach is conceptually new and combines the advantage of a simple atomic structure with high sensitivity to nuclear size.

Investigation of an electron string ion source with field emission cathode.

The string mode of operation for an electron beam ion source uses axially oscillating electrons in order to reduce power consumption, also simplifying the construction by omitting the collector with cooling requirements and has been called electron string ion source (ESIS). We have started a project (supported by INTAS and GSI) to use Schottky field emitting cathode tips for generating the electron string. The emission from these specially conditioned tips is higher by orders of magnitude than the focused Brillouin current density at magnetic fields of some Tesla and electron energies of some keV. This may avoid the observed instabilities in the transition from axially oscillating electrons to the string state of the electron plasma, opening a much wider field of possible operating parameters for an ESIS. Besides the presentation of the basic features, we emphasize in this paper a method to avoid damaging of the field emission tip by backstreaming ions.

Effect of target polarization in electron-ion recombination.

We present results of a study of the effect of target polarization on electron-ion recombination, and show that coherent radiation by the target electrons gives a large contribution to the recombination rate. It significantly modifies the nonresonant photorecombination background. A procedure has been devised whereby this contribution can be evaluated together with the conventional radiative recombination, independently of the dielectronic recombination component. Numerical results are presented for Zn2+, Cd2+, Sn4+, and Xe8+, showing up to an order-of-magnitude enhancement.