Search

Radicals produced in alanine by irradiation of 2–7 keV X-rays were investigated by ESR applications; i.e., CW-ESR, pulse-ESR and ESR imaging. The relative radical yield decreased with the decreasing photon energy and the increasing dose. The 2D imaging showed that the radical concentration at the surface was extremely low even though the absorption of X-rays exponentially decreased with the penetration depth. The depth of the radical concentration peak was almost the same as the penetration depth for the X-ray irradiation with an energy lower than 5-keV. The product formed by radical recombination was observed by an LC/MS (liquid chromatograph-mass spectrum) analysis. For the soft X-ray irradiation, many radicals will be lost by the efficient radical-radical recombination due to the higher density of the radicals, similar to the high-LET irradiation by heavy ions. The spin-spin relaxation rate (1/T 2), which correlates with the radical concentration, increased by the decreasing photon energy. The values of 1/T 2 for the soft X-rays were compatible to that for the heavy ions with the high-LET. Based on these observations, it is concluded that the effects of irradiation by soft X-rays are similar to high-LET irradiation. • The relative radical yield from soft X-rays decreased with decreasing photon energy. • The radical concentration at the surface was much lower than the energy dispersion. • The spin-spin relaxation rate increased with decreasing photon energy. • The product formed from the radical recombination was observed by LC/MS analysis. • The effects of irradiation by soft X-rays are similar to high-LET irradiation. [ABSTRACT FROM AUTHOR]

The spur reaction, a spatially nonhomogeneous chemical reaction following ionization, is crucial in radiolysis or photolysis in liquids, but the spur expansion process has yet to be elucidated. One reason is the need to understand the role of the dielectric response of the solvating molecules surrounding the charged species generated by ionization. The dielectric response corresponds to the time evolution of the permittivity and might affect the chemical reaction–diffusion of the species in a spur expansion process. This study examined the competitive relationship between reaction–diffusion kinetics and the dielectric response by solving the Debye–Smoluchowski equation while considering the dielectric response. The Coulomb force between the charged species gradually decreases with the dielectric response. Our calculation results found a condition where fast recombination occurs before the dielectric response is complete. Although it has been reported that the primary G-values of free electrons depend on the static dielectric constant under low-linear-energy transfer radiation-induced ionization, we propose that considering the dielectric response can provide a deeper insight into fast recombination reactions under high-linear-energy transfer radiation- or photo-induced ionization. Our simulation method enables the understanding of fast radiation-induced phenomena in liquids. [ABSTRACT FROM AUTHOR]

The physicochemical processes underlying the radiation sensitization of DNA by the K-shell photoabsorption of nitrogen and oxygen atoms are unclear. Accordingly, we evaluated electron paramagnetic resonance spectra of thin films of pyrimidine bases. By altering the substituents of the pyrimidine ring, we found a substituent effect on the yield of unpaired electrons induced by photoabsorption. The yield of the unpaired electrons of the DNA-base containing amino groups increased by the donation of electrons, and the yield of halogenated DNA-bases decreased by the withdrawal of electrons. These different physicochemical processes might affect DNA bases, leading to differences in DNA modification. [ABSTRACT FROM AUTHOR]

Purpose:In this paper, we review interesting findings reported in the studies of the biological effects induced by inner-shell ionization with the aim of interpreting them from a mechanistic viewpoint, in particular, of the Auger effect of atoms on different biological systems. Materials and methods:More than 70 published papers are cited on the Auger effects ranging from DNA-related elements (carbon, nitrogen, oxygen and phosphorus) to mammalian cells. Externally administrated bromine, iodine, and platinum have also been cited for the present endeavor. Those significant works all needed a highly monochromatized X-rays from brilliant synchrotron light sources. Results:We have assembled a coherent view on the inner-shell effects of the Auger process that contrasts to the overall effects with the outer-shell ionization processes. Conclusion:Some of these studies have reported that the Auger effect significantly enhances biological effects as compared with irradiation at belowK-ionization energy. The Auger-specific molecular degradation mode of DNA, involving extensive fragmentation of the deoxypentose moiety, has also been revealed. We conclude that the selectively localized effect on the specified atoms through inner-shell ionization followed by the Auger process should have a definite impact on the current radiation effect studies, which are largely based on non-selective outer-shell ionizations. [ABSTRACT FROM AUTHOR]

Purpose:To simulate the deceleration processes of secondary electrons produced by a high-energy Auger electron in water, and particularly to focus on the spatial and temporal distributions of the secondary electron and the collision events (e.g. ionization, electronic excitation, and dissociative electron attachment) that are involved in the multiplication of lesions at sites of DNA damage. Materials and methods:We developed a dynamic Monte Carlo code that considers the Coulombic force between an ejected electron and its parent cation produced by the Auger electron in water. Thus our code can simulate some return electrons to the parent cations. Using the code, we calculated to within the order of femtoseconds the temporal evolution of collision events, the mean energy, and the mean traveling distance (including its spatial probability distribution) of the electron at an ejected energy of 20 eV. Results:Some of the decelerating electrons in water in the Coulombic field were attracted to the ionized atoms (cations) by the Coulombic force within hundreds of femtoseconds, although the force did not significantly enhance the number of ionization, electronic excitation, and dissociative electron attachment collision events leading to water radiolysis. Conclusions:The secondary electrons are decelerated in water by the Coulombic force and recombined to the ionized atoms (cations). Furthermore, the some return electrons might be prehydrated in water layer near the parent cation in DNA if the electrons might be emitted from the DNA. The prehydrated electron originated from the return electron might play a significant role in inducing DNA damage. [ABSTRACT FROM PUBLISHER]

Background: The radiation-induced bystander effect is a biological response observed in non-irradiated cells surrounding an irradiated cell. The bystander effect is known to be induced by two intercellular signaling pathways, the medium-mediated pathway (MDP) and the gap junctional pathway (GJP). To investigate the relative contribution of each signaling pathway, we have developed a mathematical model of the cellular response through these two pathways, with a particular focus on cell-cycle modification. Methods: The model is based on a cellular automaton and consists of four components: (1) irradiation, (2) generation and diffusion of intercellular signals, (3) induction of DNA double-strand breaks (DSBs), and (4) cell-cycle modification or cell death. The intercellular signals are generated in and released from irradiated cells. The signals through the MDP and the GJP are modeled independently based on diffusion equations. The irradiation and both signals raise the number of DSBs, which determines transitions of cellular states, such as cell-cycle arrest or cell death. Results: Our model reproduced fairly well previously reported experimental data on the number of DSBs and cell survival curves. We examined how radiation dose and intercellular signaling dynamically affect the cell cycle. The analysis of model dynamics for the bystander cells revealed that the number of arrested cells did not increase linearly with dose. Arrested cells were more efficiently accumulated by the GJP than by the MDP. Conclusions: We present here a mathematical model that integrates various bystander responses, such as MDP and GJP signaling, DSB induction, cell-cycle arrest, and cell death. Because it simulates spatial and temporal conditions of irradiation and cellular characteristics, our model will be a powerful tool to predict dynamical radiobiological responses of a cellular population in which irradiated and non-irradiated cells co-exist. [ABSTRACT FROM AUTHOR]

The thermalization length and spatial distribution of electrons in liquid water were simulated for initial electron energies ranging from 0.1 eV to 100 keV using a dynamic Monte Carlo code. The results showed that electrons were decelerated for thermalization over a longer time period than was previously predicted. This long thermalization time significantly contributed to the series of processes from initial ionization to hydration. We further studied the particular deceleration process of electrons at an incident energy of 1 eV, focusing on the temporal evolution of total track length, mean traveling distance, and energy distributions of decelerating electrons. The initial prehydration time and thermalization periods were estimated to be approximately 50 and 220 fs, respectively, indicating that the initial prehydration began before or contemporaneously with the thermal equilibrium. Based on these results, the prehydrated electrons were suggested to play an important role during multiple DNA damage induction. [ABSTRACT FROM AUTHOR]

We calculated cross sections, stopping powers, and energy loss rates for rotational and phonon-mode excitations caused by the impact of an electron in an energy region from 0.1 meV to 100 eV injected into liquid water. The spatial distribution of the decelerated electron depends on these cross sections. We performed calculations assuming an optical approximation with the dielectric functions that are experimentally reported in the literature. We observed that the cross sections lie below 1×10 −16 cm 2 over the considered energy region. The values for rotational excitation processes in the liquid phase are less by three orders of magnitude than those in the gas phase because of the screening effect of neighboring water molecules on the interaction between the incident electron and water molecules. These results suggest that the cross sections in the liquid phase are significantly different from those in the gas phase. The values for phonon-mode excitations in the liquid phase are close to those reported for amorphous ice. Furthermore, we observed that the stopping power shows a maximum around 200 meV, and the energy loss rates, which are derived from the stopping power, depend significantly on the electron energy, particularly below 1 eV. The values obtained here will allow us to precisely estimate the decelerating process of an electron in liquid water to predict radiation effects such as chemical processes in water radiolysis or biomolecular damage induction strongly involved in low energy electron processes. [ABSTRACT FROM AUTHOR]

Highlights: [•] Dynamical calculations of electrons in water were performed. [•] Effect of Coulomb interaction between charged particles was investigated. [•] About 20% of the ionized electrons were returned to the ions within 100fs. [Copyright &y& Elsevier]

Histone proteins, building blocks of chromatins, participate in enzymatic reactions in cells heated at around 45°C though in vitro the denaturation of histones significantly proceeds at a similar temperature. It implies that unidentified mechanisms prevent thermal denaturation of histones in vivo. However, studies on the histone structures in the heated cells have been scarce. Here, we analyzed the secondary structures of histone H2A‐H2B proteins originating from the heated cells using circular dichroism spectroscopy. The secondary structure contents of the H2A‐H2B extracted from the heated cells differed from those of H2A‐H2B both native and denatured in vitro but reverted to the native structures by incubating the heated cells at 37°C within 2 h. Such structural flexibility may play a role in protecting genomic functions governed by chromatin structures from heat stresses. [ABSTRACT FROM AUTHOR]

Purpose: To study the lifetime of the short-lived unpaired electron species induced in a calf thymus DNA film by soft X-rays in the energy regions around nitrogen and oxygen K-edge. Materials and methods: Dry calf thymus DNA films were measured by electron paramagnetic resonance (EPR) spectrometer during the monochromatic soft X-ray irradiation provided from a synchrotron accelerator (SPring-8, Super Photon ring-8 GeV). Two operation modes of SPring-8 with different electron bunch structures were used to explore dependence of EPR intensity on pulse interval of soft X-rays. Results: Observed EPR spectra did not show significant difference, in the spectral shape, intensity, g-factor and photon energy dependence, between the two bunch modes. Conclusions: The total yield of the unpaired electron species were estimated by assuming the exponential relaxation of the unpaired electrons with two different bunch modes of the synchrotron. The lifetime of the unpaired electron species is estimated to be in the range of a few tens to several hundreds of μs. [ABSTRACT FROM AUTHOR]

Abstract: The X-ray absorption of nucleotides (adenosine-5′-monophosphate, guanosine 5′-monophosphate, and cytidine 5′-monophosphate) are measured in both water solutions and thin solid films at X-ray energies near the nitrogen K-edge in the ‘water-window’ region. Each spectrum corresponds to the selective excitation of a nucleobase site in a nucleotide, and thus has features similar to the spectrum of the corresponding nucleobase. An additional new peak in the energy region of the nitrogen 1s→π* resonance is observed for each nucleotide. No significant difference between the water solutions and thin solid films is found, which might be attributable to the hydrophobic properties of a nucleobase in a nucleotide. [Copyright &y& Elsevier]

Abstract: To reveal the chemical changes induced in DNA by irradiation with ionizing radiation, we have investigated the spectral change in the X-ray absorption near edge structure (XANES) of DNA resulting from exposure to monochromatic soft X-rays. We used a thin film of calf thymus DNA as a sample and observed nitrogen K-shell and oxygen K-shell XANES spectra. The typical monochromatic soft X-ray energies used for the irradiation (395, 408, 528, and 538eV) were obtained from a soft X-ray beamline (BL23SU, SPring-8). These energies correspond to those just below or just above the nitrogen and oxygen K-shell ionization energy, respectively. The obtained XANES spectra show significant changes by irradiation. Particularly a new π* resonance peak in oxygen XANES spectra evidently appeared by the irradiation above oxygen K-shell ionization potential. These results suggest that carbonyl groups, presumably a propenal group (OCe produced in the sample by oxygen ionization. Thus characteristic damage induced by induction in the DNA molecule would be predicted following exposure to monochromatized synchrotron soft X-rays. [Copyright &y& Elsevier]

Abstract: A new spectroscopy to identify the hydration structure playing important role in liquid-phase radiation damage is in progress using a laminar liquid water jet sample in vacuum in combination with soft X-ray synchrotron radiation. We present the total and partial electron yields for liquid water using a photoelectron spectroscopy. Partial electron yields for the Auger transition are obtained for the first time by measuring the electrostatically dispersed electron kinetic energy spectra as a function of photon energy of synchrotron radiation. [Copyright &y& Elsevier]

Purpose: To reveal the reaction process in DNA by Auger electrons using the observed single strand breaks (SSB) and base lesions induced by monochromatic ultrasoft X-rays in dry plasmid DNA film. Desorbed ions from DNA and 2-deoxy-D-ribose thin films were also measured to ascertain the decomposition site in DNA. Materials and methods: Dry plasmid DNA (pUC18) films were irradiated with synchrotron monochromatic ultrasoft X-rays (USX). Two photon energies, 270, and 560 eV, were chosen for the irradiation experiments. Irradiated plasmid DNA was analyzed by agarose gel electrophoresis. The yield of base lesions was determined by the post-irradiation-treatment of the DNA with enzymatic probes (formamidpyrimidine DNA glycosylase [Fpg] and endonuclease III [Nth]). Desorbed ions induced by 540 eV USX irradiation from calf thymus DNA and 2-deoxy-D-ribose thin films were detected by quadrupole-mass spectrometer. Results: Yield of strand breaks and base lesions were obtained by 270 and 560 eV photon energies, respectively. Each yield showed characteristic of the photon energy spectrum. The characteristics of the desorbed ion mass spectra from 2-deoxy-D-ribose and DNA films were strikingly similar with each other. Conclusions: In this paper we report, for the first time, the yields of base lesions and SSB induced by monochromatic USX. The yield of SSB induced by core-ionization of carbon, nitrogen, and oxygen was two times more frequent than that of valence-electrons. From the comparison of desorbed ion mass spectra of 2-deoxy-D-ribose with DNA films we predict these breaks are likely to be induced by the decomposition of the sugar sites in DNA backbone. [ABSTRACT FROM AUTHOR]

Abstract: A new spectroscopy for direct effect of radiation damage to nucleic acids such as DNA and RNA is underway using a liquid beam sample in vacuum combined with soft-X-ray synchrotron radiation. We show the X-ray absorption spectrum (XANES) of liquid phase water at X-ray photon energy in the vicinity of oxygen K-shell absorption edge obtained from total photoelectron yields ejected from a pure water beam. We confirm a “liquid sample in vacuum” for the present experiment by the measurements of the temperature dependence of the XANES spectrum for a liquid beam of pure water. Shown is the first measurement of the XANES spectrum for guanosine--monophosphate (GMP), which is one of the fundamental nucleotide unit for RNA, in water solution at X-ray photon energy in the vicinity of nitrogen K-shell absorption edge involved in the ‘water-window’ region, which corresponds to a selective excitation of guanine site. [Copyright &y& Elsevier]

Purpose: To study DNA strand breaks induced by direct energy deposition by photo- and Auger electrons ejected following K-shell photoabsorption of DNA constituent atoms (carbon, nitrogen, oxygen and phosphorus). Method: Using a Monte Carlo code which has been developed to simulate the photoelectric effect on plasmid DNA pBR322, the energy deposition pattern of secondary electrons ejected after photoabsorption in DNA constituent atoms (not including the hydration shell) was calculated. Experimentally obtained X-ray absorption near edge structures were considered of the cross-sections at the K-shell resonant absorption (1s&rarrα*) of carbon, nitrogen and oxygen, and the K-shell resonant absorption (1s→α*) of phosphorus. Direct energy deposition by secondary electrons was scored using two different DNA models with and without the hydrated shell. The yields of SSB, DSB per photoabsorption events as well as break complexity were estimated for monochromatic X-rays around the K-edges of DNA constituent atoms (200-3000 eV). Result: Higher SSB and DSB yields were obtained below the carbon, nitrogen and oxygen K-edges compared to at or above the resonance, and at the K-shell resonant absorption of phosphorus compared to below the resonance. The number of electrons with sufficient energy to induce strand breaks was found to change depending on the photon energy. Electrons with 120 and 250 eV are shown to be rather more effective in SSB and DSB induction than electrons with higher energy. Inclusion of hydrated water in the DNA volume did not affect the photon energy dependence of the strand break yields. Conclusion: The small difference of photon energies around K-absorption edges of the carbon, nitrogen, oxygen and phosphorus is indicated to induce variation in strand break yields by direct effect. Higher SSB and DSB induction efficiencies could be due to a higher yield of more than two electrons with around 120 eV to 250 eV per photoabsorption event. [ABSTRACT FROM AUTHOR]

Purpose: In order to understand the molecular mechanism of nucleobase damage caused by ultrasoft X-ray irradiation, guanine radicals have been studied using an X-band EPR (electron paramagnetic resonance) spectrometer installed in a synchrotron soft X-ray beamline. Materials and Methods: Guanine pellets were irradiated under vacuum with ultrasound X-rays obtained from a soft X-ray beamline (BL23SU) in SPring-8. The energy regions around the nitrogen (0.4 keV) and oxygen (0.5 keV) K-edges were chosen for the irradiation. The ultrasoft X-ray irradiation and EPR measurements were carried out simultaneously at low temperature, 20 K and 77 K. Results: The KFR spectrum observed during irradiation was clearly distinguishable from that of the stable radical, which still exists after exposure to ulrasoft X-rays at 77 K. The spectrum of the short-lived radicals consisted of two components, which exhibited different EPR microwave power saturation. The EPR signal intensities increased linearly with increasing dose rate (photon flux density). These signals immediately disappeared when the beam was turned off, even when irradiated at lower temperature (20 K). At the energy of the oxygen K-resonance excitation (539 eV) the signal intensity was clearly increased to more than five times that obtained on the lower energy side (526 eV). On the other hand, the enhancement was insignificant above and below the nitrogen K-edge (401 eV). The singlet EPR signal of the stable radical was similar to that reported previously in the literature for γ-irradiated guanine. Conclusions: The short-lived radical species observed were mainly induced as a result of the final state of the resonant Auger process on oxygen atoms existing solely in the carbonyl group in guanine. Auger events at the other atoms in guanine (namely, carbon and nitrogen) do not induce this radical process to any great extent, even though the abundance of these atoms (i.e. the sum of their photoabsorption cross sections) is dominant in the guanine molecule. [ABSTRACT FROM AUTHOR]

Examines the effect of hydration on the induction of strand breaks and base lesions in plasmid DNA films by gamma-radiation. Quantification of the yields in strand breaks in irradiated plasmid DNA; Detection of enzyme-sensitive DNA damage; Dependence of the yield of ssb on the level of hydration; Yield of addition dsbs revealed by treatment with nth and Fpg.

In this work, we investigate the physicochemical process of water photolysis to bridge physical and chemical processes by a newly developed first-principles calculation code. The deceleration, thermalization, delocalization, and initial hydration of the extremely low-energy electrons ejected by water photolysis are sequentially tracked in the condensed phase. We show herein the calculated results for these sequential phenomena during 300 fs. Our results indicate that the mechanisms heavily depend on the intermolecular vibration and rotation modes peculiar to water and the momentum transfer between the electrons and the water medium. We suggest that using our results for the delocalized electron distribution will reproduce successive chemical reactions measured by photolysis experiments using a chemical reaction code. We expect our approach to become a powerful technique for various scientific fields related to water photolysis and radiolysis. [ABSTRACT FROM AUTHOR]

The article discusses the highlights of the 8th International Symposium on Physical, Molecular, Cellular, and Medical Aspects of Auger Processes held in Kyoto, Japan in May 2015. Speakers at the event included Roger Martin, who explored developments in the clinical translation of Auger research, and Katherine Vallis, who discussed the applications of Auger emitters in cancer therapy. Information on the history of Auger symposia is also provided.

To investigate the structural features of wild-type and phospho-mimicking mutated XRCC4 protein, a protein involved in DNA double-strand break repair. XRCC4 with a HisTag were expressed by E. coli harboring plasmid DNA and purified. Phospho-mimicking mutants in which one phosphorylation site was replaced with aspartic acid were also prepared in order to reproduce the negative charge resulting from phosphorylation. The proteins were separated into dimers and multimers by gel filtration chromatography. Circular dichroism (CD) spectroscopy was performed in the region from ultraviolet to vacuum-ultraviolet. The CD spectra were analyzed with two analysis programs to evaluate the secondary structures of the wild-type and phospho-mimicked dimers and multimers. The proportion of β-strand in the wild-type dimers was very low, particularly in their C-terminal region, including the five phosphorylation sites. The secondary structure of the phospho-mimic hardly changed in the dimeric form. In contrast, the β-strand content increased and the α-helix content decreased upon multimerization of the wild-type protein. The structural change of multimers slightly depended on the phospho-mimic site. These results suggest that the β-strand structure stabilizes the multimerization of XRCC4 and it is regulated by phosphorylation at the C-terminal site in living cells. An increase in the β-strand content in XRCC4 is essential for stabilization of the multimeric form through C-terminal phosphorylation, allowing the formation of the large double-strand break repair machinery. [ABSTRACT FROM AUTHOR]

Purpose:To discuss low-dose radiation-induced risks to male fertility focusing on potential mechanisms of low-dose radiation-induced damage on spermatogenesis, epidemiological studies of environmental radiation effects on sperm parameters and transgenerational effects following exposure of spermatogonial stem cells (SSCs). Background:Spermatogenesis produces mature male gametes, spermatozoa, which fertilize their counterpart female gametes, oocytes. The robust maintenance system of spermatogenesis is essential for genomic conservation; however, male fertility can be readily impacted by exposure to environmental, chemical and physical factors including ionizing radiation. The mammalian testes are known to be radiosensitive yet the underlying molecular mechanisms of low-dose radiation-induced risks for spermatogenesis remain unclear. Furthermore, evidence characterizing transgenerational effects following exposure of SSCs remain controversial. Conclusions:Current concerns over the possible effects of low-dose radiation exposure on spermatogenesis requires further elucidation that may be resolved comparing and integrating observed experimental and epidemiological data. [ABSTRACT FROM PUBLISHER]

Irreversible cell-cycle-arrested cells not undergoing cell divisions have been thought to be metabolically less active because of the unnecessary consumption of energy for cell division. On the other hand, they might be actively involved in the tissue microenvironment through an inflammatory response. In this study, we examined the mitochondria-dependent metabolism in human cells irreversibly arrested in response to ionizing radiation to confirm this possibility. Human primary WI-38 fibroblast cells and the BJ-5ta fibroblast-like cell line were exposed to 20 Gy X-rays and cultured for up to 9 days after irradiation. The mitochondrial morphology and membrane potential were evaluated in the cells using the mitochondrial-specific fluorescent reagents MitoTracker Green (MTG) and 5,5′,6,6′-tetraethyl-benzimidazolylcarbocyanine iodide (JC-1), respectively. The ratio of the mean MTG-stained total mitochondrial area per unit cell area decreased for up to 9 days after X-irradiation. The fraction of the high mitochondrial membrane potential area visualized by JC-1 staining reached its minimum 2 days after irradiation and then increased (particularly, WI-38 cells increased 1.8-fold the value of the control). Their chronological changes indicate that the mitochondrial volume in the irreversible cell-cycle-arrested cells showed significant increase concurrently with cellular volume expansion, indicating that the mitochondria-dependent energy metabolism was still active. These results indicate that the energy metabolism in X-ray-induced senescent-like cells is active compared to nonirradiated normal cells, even though they do not undergo cell divisions. [ABSTRACT FROM AUTHOR]

To clarify the radiosensitization mechanism masking the Auger effect of the cells possessing brominated DNA, the electronic properties of DNA-related molecules containing Br were investigated by X-ray spectroscopy and specific heat measurement. X-ray absorption near-edge structure (XANES) and X-ray photoemission spectroscopy (XPS) were used to measure the electronic properties of the nucleotides with and without Br. We determined the specific heat of 5-bromouracil crystals with thymine as a reference molecule at low temperatures of 3–48 K to calculate the microscopic state numbers. Obtained XANES and XPS spectra indicated that both the lowest unoccupied molecular orbital (LUMO) and the core-levels were not affected by the Br incorporation. The state numbers of 5-bromouracil calculated from the specific heats obtained around 25 K was about 1.5 times larger than that for thymine below 20 K, although the numbers were almost the same below 5 K. Our results suggest that the Br atom may not contribute substantially to the LUMO and core-level electronic states of the molecule, but rather to the microscopic states related to the excitation of lattice vibrations, which may be involved in valence electronic states. [ABSTRACT FROM AUTHOR]

To identify the bonding sites of initial radiation interaction with DNA and to trace the following chemical reaction sequences on the pathway of damage induction, we carry out a spectroscopy XIL (X-ray induced luminescence) using soft X-ray synchrotron radiation. This is a nondestructive analysis of the excited intermediate species produced in a molecular mechanism on the damage induction pathway. We introduce aqueous samples of UMP (uridine-5'-monophosphate) in the vacuum by the use of a liquid micro-jet technique. The luminescence in the region of UV-VIS (from visible to ultraviolet) radiation induced after the absorption of monochromatic soft X-ray by aqueous UMP is measured with sweeping the soft X-ray energy in the region of 370–560 eV. The enhanced XIL intensities for aqueous UMP in the region of soft X-ray of 410–530 eV (in "water window" region) are obtained. The enhancement of XIL intensities in the UV-VIS region, relative to the water control, is explained by the excitation and ionization of a K-shell electron of nitrogen atoms in the uracil moiety. The enhanced XIL intensities do not match the structure of XANES (X-ray absorption near-edge structure) of the aqueous UMP. This suggests that the XIL intensities reflect the quantum yields of luminescence, or the quantum yields for conversion by UMP of an absorbed X-ray into UV-VIS radiation. In this paper, spectra of luminescence are shown to be resolved by combining low pass filters. The filtered luminescence spectra are obtained at the center of gravity (λc) of the band pass wavelength regions at λc = 270nm, 295 nm, 340 nm, 385 nm, 450 nm, and 525 nm., which show a trend similar to the fluorescence of nucleobases induced by ultraviolet radiation. It is concluded that the origin of the observed XIL is the hydrated uracil moiety in aqueous UMP, decomposition of which is suppressed by the migration of excess charge and internal energy after the double ionization due to Auger decay. [ABSTRACT FROM AUTHOR]

Highlights: [•] We transferred a human chromosome exposed to UV-A into unirradiated mouse cells. [•] The transferred chromosome exposed to UV-A induced genetic instability. [•] Instability occurred not only in the exposed chromosome but also in the unirradiated cells. [•] The unstable chromosome harboring DNA damage by UV-A may destabilize unirradiated chromosomes. [Copyright &y& Elsevier]

In clinical settings, the development of novel X-ray-activated caged compounds has been described as a key factor in both therapeutic and diagnostic applications to halt the action of specific medication, i.e., reactivation by external X-ray irradiation causing site-selective bond breakages, during delivery to target organs. In this work, we focused on a hypervalent iodine compound, namely 2-iodosobenzoic acid (IBA), as a candidate for the protecting group of X-ray-activated caged compounds, and investigated the decomposition reactions of IBA induced by X-ray irradiations around the iodine L 3 -edge energy region. Our results showed that X-irradiations induced bond breakages around the hypervalent iodine atom, and the associated decomposition rate was dependent on the irradiated X-ray photon energy. Particularly, X-ray absorption spectral changes and desorbed molecular fragments confirmed a significant molecular decomposition induced by one of the iodine L-shell excitations. Our findings suggest that the hypervalent iodine compound could be a very promising candidate for the design of novel X-ray-activated caged compounds. • Monochromatic X-ray irradiation induced bond breakages around the hypervalent iodine atom in IBA. • Specific resonant excitation (I 2p 3/2.→ π*) significantly induced the decomposition of IBA. • The IBA is one of the key candidates for the protecting group of novel X-ray-activated caged compounds. [ABSTRACT FROM AUTHOR]

To apply a hypervalent iodine compound, 2-iodosobenzoic acid (IBA), to a protecting group of X-ray activated caged compounds, X-ray photoemission and absorption spectroscopy of IBA were performed. The photoemission spectrum of IBA in the valence band region included a component originating from the π*-like orbitals of I-C and I-O. Such characteristic electronic structures may cause the reactivities of IBA. The linear absorption coefficients of the peaks in the iodine K -edge energy region were 38.8, 40.0, and 39.41 cm−1 at around 33.18, 33.23, and 33.32 keV, respectively. Those contribute to discussing reactions of IBA induced by the iodine K -shell excitations quantitatively. [ABSTRACT FROM AUTHOR]

Purpose: To investigate which type of bond is more likely to be cleaved in functional groups in DNA including the nucleobases by the K-shell ionization of oxygen of DNA, and to determine whether the production of propenal is specific to the oxygen resonant excitation. To investigate the degradation pattern which depends on the type of nucleobase in the DNA monomer. Materials and methods: Calf thymus DNA film and four nucleotides (dAMP, TMP, dGMP, and dCMP) films were used as samples. Soft X-rays with energy of 560 eV were used to irradiate the samples, and the changes in the X-ray absorption near edge structure (XANES) spectra during the irradiation were measured. The XANES measurements were performed by using a 0.02 eV scanning photon energy step. Results: The difference spectra for DNA and nucleotides were similar to those for pyridine deprotonation. The oxygen K-edge regions in the difference spectra were all similar apart from the spectrum obtained at the resonant excitation energy of oxygen in DNA. The spectral change did not depend on the type of nucleotide. Conclusion: (1) Deprotonation of the nucleobase -NH is usually induced by core ionization of carbon, nitrogen, and oxygen; (2) propenal production is specific to the oxygen K-shell resonant excitation; and (3) the pattern of XANES spectral changes does not significantly depend on the type of nucleobase. [ABSTRACT FROM AUTHOR]

Abstract: The biological consequences of clusters containing a single strand break and base lesion(s) remain largely unknown. In the present study we determined the mutagenicities of two- and three-lesion clustered damage sites containing a 1-nucleotide gap (GAP) and 8-oxo-7,8-dihydroguanine(s) (8-oxoG(s)) in Escherichia coli. The mutation frequencies (MFs) of bi-stranded two-lesion clusters (GAP/8-oxoG), especially in mutY-deficient strains, were high and were similar to those for bi-stranded clusters with 8-oxoG and base lesions/AP sites, suggesting that the GAP is processed with an efficiency similar to the efficiency of processing a base lesion or an AP site within a cluster. The MFs of tandem two-lesion clusters comprised of a GAP and an 8-oxoG on the same strand were comparable to or less than the MF of a single 8-oxoG. The mutagenic potential of three-lesion clusters, which were comprised of a tandem lesion (a GAP and an 8-oxoG) and an opposing single 8-oxoG, was higher than that of a single 8-oxoG, but was no more than that of a bi-stranded 8-oxoGs. We suggest that incorporation of a nucleotide opposite 8-oxoG is less mutagenic when a GAP is present in a cluster than when a GAP is absent. Our observations indicate that the repair of a GAP is retarded by an opposing 8-oxoG, but not by a tandem 8-oxoG, and that the extent of GAP repair determines the biological consequences. [Copyright &y& Elsevier]

Purpose: To study the DNA lesions induced by the Auger decay of carbon, nitrogen, and oxygen using ultrasoft X-rays (USX) that are expected to be important with the DNA repair system of living cells. Materials and methods: pUC19 plasmid DNA dry samples were irradiated with USX photons at 270 and 560 eV and 60Co γ-rays in vacuum at room temperature. The amounts of unaltered base release by the direct radiation effects were quantified using high-performance liquid chromatography. To quantify and characterize the strand break termini the rate at which snake venom phosphodiesterase (SVPD) digested irradiated DNA pretreated with and without calf intestine alkaline phosphatase was measured. Moreover, the piperidine-labile base lesions and abasic sites of the irradiated DNA were estimated using the SVPD method. Results: The yields of unaltered base release for 270, 560 eV photons and 60Co γ-rays were 0.016, 0.014, and 0.018 μmol/J, respectively. The total 3' termini for the three kinds of photons were around 0.1 μmol/J. The production of 3' termini with phosphate was found to be predominant with respect to that of 3'OH termini for the three kinds of radiation. The yield of piperidine-labile sites for 270 eV (∼0.1 μmol/J) was slightly larger than that for 560 eV (∼0.07 μmol/J) and also for γ-rays (∼0.082 μmol/J). Conclusion: Although the Auger process in DNA-constituent atoms was expected to induce Auger-specific lesions in the molecule the chemical endpoints would have been covered with a large number of lesions produced from secondary electrons in the surrounding bulk DNA molecules. The present results, however, suggest that a low-energy electron field produced by the USX photons in the bulk DNA is basically not at all specific to DNA damage being produced when compared with the high-energy electron field produced by 60Co γ-rays. [ABSTRACT FROM AUTHOR]

Purpose: To compare the molecular decomposition of 2-deoxy-D-ribose induced by 0.5 keV ultrasoft X-ray irradiation. Materials and methods: A thin film of 2-deoxy-D-ribose was irradiated by two radiation sources: low-energy (∼ 0.6 keV) electrons and ultrasoft X-rays (∼ 0.5 keV). The positive ions that were desorbed from the sample during the irradiation were measured using a quadrupole mass spectrometer. The spectral changes in the X-ray absorption near edge structure (XANES) were also examined after the irradiation. Results and discussion: The ions that were desorbed from 2-deoxy-D-ribose due to electron irradiation were mainly H+, CHx+, C2Hx+, CO+, CHxO+, C3Hx+, C2HxO+ and C3HxO+ (x = 1, 2, and 3) ions. These ions were the same as those observed in desorption due to ultrasoft X-ray irradiation. The XANES spectral changes induced by electron irradiation showed C-O bond cleavage in the molecule and C = O bond formation in the surface residues. These results show that intensive molecular decomposition of the furanose ring structure was induced by both types of irradiation. It is inferred that these irradiation products are primarily produced by secondary electrons (several tens of eV), which are thought to be generated by both types of irradiation when they are applied to the 2-deoxy-D-ribose sample. [ABSTRACT FROM AUTHOR]

Purpose: To investigate quantitatively and qualitatively the production of thymine radicals produced by monochromatic ultrasoft X (USX) or 60Co γ-rays using electron paramagnetic resonance (EPR) spectroscopy. Materials and Methods: Thymine was chosen as the DNA component for the irradiation. The EPR experiments of irradiated thymine were performed using an X-band EPR device installed in a soft X-ray beamline (BL23SU) in SPring-8. Sample pellets were irradiated with USX photons in a microwave cavity in a vacuum chamber. EPR measurements of thymine powder pellets irradiated with USX photons at energies of 407 and 538 eV were performed at 77 K or room temperature. For reference, 60Co γ-irradiation to a pellet was also performed at room temperature. Results: The following three features were found: 1) comparison between the two energies shows that the EPR dose-response curves are clearly distinguishable from each other: the curve for 407 eV saturated at a lower dose and spin number than that for 538 eV, 2) no evident qualitative difference between the radical species produced at the two energies was observed, 3) the EPR signal of the 538 eV USX-irradiated sample measured after annealing for 12 days is similar to that obtained with 60Co γ-irradiation. Conclusions: The difference observed in the EPR dose-response relationship reflects the difference in the K-absorption cross-sections of carbon, nitrogen and oxygen in the thymic molecule which govern the photo-/Auger electron energy spectrum. [ABSTRACT FROM AUTHOR]

The N K-edge X-ray absorption near edge structure (XANES) spectra of the purine-containing nucleotide, guanosine 5'-monophosphate (GMP), in aqueous solution are measured under various pH conditions. The spectra show characteristic peaks, which originate from resonant excitations of N 1s electrons to n * orbitals inside the guanine moiety of GMP. The relative intensities of these peaks depend on the pH values of the solution. The pH dependence is explained by the core-level shift of N atoms at specific sites caused by protonation and deprotonation. The experimental spectra are compared with theoretical spectra calculated by using density functional theory for GMP and the other purine-containing nucleotides, adenosine 5'-monophosphate, and adenosine 5'-triphosphate. The N K-edge XANES spectra for all of these nucleotides are classified by the numbers of N atoms with particular chemical bonding characteristics in the purine moiety. [ABSTRACT FROM AUTHOR]

[Display omitted] • X-ray dissociation of tin (IV) phthalocyanine dichloride was investigated. • X-ray irradiation induced ligand cleavages, namely, desorption of chlorine atoms. • Tin phthalocyanine derivatives can be used as X-ray-activated caged compounds. The X-ray dissociation of tin (IV) phthalocyanine dichloride (SnCl 2 Pc) was investigated using X-ray photoemission spectroscopy. The photoemission spectra of SnCl 2 Pc after X-ray irradiation exhibited additional components assigned as chloride ions and tin (II) phthalocyanine, as well as decomposition products from the phthalocyanine framework, indicating that X-ray irradiation induced the release of chloride ions as ligands from SnCl 2 Pc. Although the ligand releases were caused not by a simple cleavage between Cl-Sn bonding but by accompanying the molecular framework destruction, these reactions may robustly facilitate a novel application of X-rays to activate caged compounds in tissues. [ABSTRACT FROM AUTHOR]

Purpose:To clarify whether initial base excision repair processes at clustered DNA damage sites comprising multiple base lesions affect subsequent excision processes via the formation of additional strand breaks by glycosylase and apurinic/apyrimidinic (AP) endonuclease base excision enzymes. Materials and methods:Plasmid DNA (pUC18) as a model DNA molecule was exposed to high-linear-energy-transfer (LET) ionizing radiation (He2+ or C6+ ions) or low-LET ionizing radiation (X-rays) under various conditions to produce varied radical-scavenging effects. pUC18 was then treated sequentially or simultaneously with two bacterial base excision enzymes (glycosylases), namely, endonuclease III and formamidopyrimidine-DNA glycosylase, which convert pyrimidine (or abasic [AP] site) and purine (or AP site) lesions to single-strand breaks (SSB), respectively. Yields of additional SSB or double-strand breaks (DSB) as digestion products were examined after changing the order of enzymatic treatment. Results:There were few differences among the enzymatic treatments, indicating that treatment order did not affect the final yields of additional SSB or DSB formed by glycosylase activity. This suggests that of the total damage, the fraction of clustered damage sites with a persistent base lesion dependent on the order of glycosylase treatment was insignificant if present. Conclusion:Base lesion clusters induced by high- or low-LET radiation appear three or more base pairs apart, and are promptly converted to a DSB by glycosylase, regardless of the order of enzymatic treatment. [ABSTRACT FROM PUBLISHER]

Purpose:To establish a new experimental technique to explore the photoelectric and subsequent Auger effects on the cell cycles of soft X-ray microbeam-irradiated cells and unirradiated bystander cells in a single colony. Materials and methods:Several cells located in the center of a microcolony of HeLa-Fucci cells consisting of 20–80 cells were irradiated with soft X-ray (5.35 keV) microbeam using synchrotron radiation as a light source. All cells in the colony were tracked for 72 h by time-lapse microscopy imaging. Cell cycle progression, division, and death of each cell in the movies obtained were analyzed by pedigree assay. The number of cell divisions in the microcolony was also determined. Results:The fates of these cells were clarified by tracking both irradiated and unirradiated bystander cells. Irradiated cells showed significant cell cycle retardation, explosive cell death, or cell fusion after a few divisions. These serious effects were also observed in 15 and 26% of the bystander cells for 10 and 20 Gy irradiation, respectively, and frequently appeared in at least two daughter or granddaughter cells from a single-parent cell. Conclusions:We successfully tracked the fates of microbeam-irradiated cells and unirradiated bystander cells with live cell recordings, which have revealed the dynamics of soft X-ray irradiated and unirradiated bystander cells for the first time. Notably, cell deaths or cell cycle arrests frequently arose in closely related cells. These details would not have been revealed by a conventional immunostaining imaging method. Our approach promises to reveal the dynamic cellular effects of soft X-ray microbeam irradiation and subsequent Auger processes from various endpoints in future studies. [ABSTRACT FROM PUBLISHER]

Purpose:To investigate an enhancement of DNA double-strand break (DSB) induction and cell killing effect by K-shell ionization of phosphorus atoms and Auger electrons on human cell lines. Materials and methods:Induction of DSB, DNA damage responses, cell cycle distributions, and cell killing effects were investigated after exposures of the cells with monochromatic synchrotron radiation soft X-rays of 2153 and 2147 eV, which were the resonance peak and off peak, respectively, of the K-shell photoabsorption of phosphorus. Results:Higher biological effects in the cells irradiated with soft X-rays at 2153 eV than at 2147 eV were observed in (i) the efficiency of 53BP1/γ-H2AX co-localized foci formation per dose and residual number of foci, (ii) prolonged phosphorylation levels of DSB repair and/or cell cycle checkpoint related proteins and G2 arrest, (iii) the cell killing effects at the 10% survival level of normal human fibroblasts, HeLa cells, and human glioblastoma M059K cells (1.2–1.5 times higher) and that of human ataxia telangiectasia mutated (ATM)-defective cells and glioblastoma DNA-dependent protein kinase catalytic subunit (DNA-PKcs)-defective cells (1.2 times). Conclusion:The yield of DSB and partly less-reparable complex DNA damage induction in human cells was enhanced by K-shell photoabsorption of phosphorus and low-energy Auger electrons. [ABSTRACT FROM PUBLISHER]

[Display omitted] • The SAC-CI study on the XPS spectra of Thymine (T) and 5-bromouracil (BrU) measured in the wide range of 0–2000 eV by Yokoya and co-workers. • Theoretical SAC-CI ionization spectra agreed with experiments to within 1.0 eV for both T and BrU. • The inner-core ionization processes involve many electronic processes like ionizations, and those accompanying excitations, reorganizations, and electron correlations. • Accurate calculations require both good theory and good basis set. • The present SAC-CI calculations with extended bases including even Rydberg orbitals have given reliable results. The X-ray Photoelectron Spectra (XPS) of thymine (T) and 5-Bromouracil (BrU) were studied and assigned using Symmetry-Adapted-Cluster Configuration-Interaction (SAC-CI) theory. The differences between theory and experiment were within 1.0 eV when compared with the spectra measured in vacuum conditions. It was clarified that inner-core ionization processes involve many electronic processes like ionizations, accompanying excitations, reorganizations, and electron correlations. Therefore, accurate calculations require both good theory and good basis set. The present SAC-CI calculations with extended basis sets including even Rydberg basis functions may satisfy these requirements. [ABSTRACT FROM AUTHOR]

Radicals produced in crystalline L-alanine-3,3,3-d3 and L-alanine-d4 were observed by the electron spin resonance (ESR) technique during 1.5 keV soft X-ray irradiation. The spectral change from •CHCD 3 COOH to •CDCD 3 COOH by a hydrogen exchange reaction was directly observed in L-alanine-3,3,3-d3. The line width of the ESR spectra obtained by the soft X-ray irradiation was 1.5 times wider than that of hard X-rays from a previous report, meaning a higher density of radicals. The efficiency of the radical yield by the soft X-ray irradiation was 10−6 relative to that by γ-irradiation. The product formed by the radical recombination was observed by an LC/MS analysis. For the soft X-ray irradiation, many radicals will be lost by the efficient radical-radical recombination due to the higher density of the radicals, just as the high-LET irradiation by heavy ions. The result was the same as the yield of hydroxyl radicals in an aqueous solution that decreased with the decreasing photon energy from 1.25 MeV to 1.5 keV. We concluded that the high LET nature of the lower energy photons leads to the dense radical formation in the crystalline alanine powder. • The line width of ESR spectra by soft X-ray irradiation was wider than hard X-ray. • The yield of radical by soft X-ray irradiation was much lower than γ-irradiation. • The product formed by the radical recombination was observed by an LC/MS analysis. • The high LET nature of the lower energy photons leads to the dense radical formation. [ABSTRACT FROM AUTHOR]

Highlights: [•] XANES for adenine nucleotides in aqueous solutions at N K-edge are measured. [•] XANES spectra showed pH value dependence. [•] The pH value dependence is explained as due to protonation of N1 atom in adenine. [•] This explanation is consistent with the published results for gas phase adenine. [ABSTRACT FROM AUTHOR]

Highlights: [•] We examined the role of Pol I in processing clustered damage sites in vivo. [•] Efficient transformation of bi-stranded GAP/AP cluster requires Pol I activity. [•] Pol I plays an important role in minimising clustered lesion-induced mutations. [•] The biological consequences depend on the extent of repair of the strand breaks. [•] Pol I may be required in a lesion-avoidance pathway during replication. [ABSTRACT FROM AUTHOR]

The non-targeted effects of radiation have been known to induce significant alternations in cell survival. Although the effects might govern the progression of tumor sites following advanced radiotherapy, the impacts on the intercellular control of the cell cycle following radiation exposure with a modified field, remain to be determined. Recently, a fluorescent ubiquitination-based cell-cycle indicator (FUCCI), which can visualize the cell-cycle phases with fluorescence microscopy in real time, was developed for biological cell research. In this study, we investigated the non-targeted effects on the regulation of the cell cycle of human cervical carcinoma (HeLa) cells with imperfect p53 function that express the FUCCI (HeLa–FUCCI cells). The possible effects on the cell-cycle phases via soluble factors were analyzed following exposure to different field configurations, which were delivered using a 150 kVp X-ray irradiator. In addition, using synchrotron-generated, 5.35 keV monochromatic X-ray microbeams, high-precision 200 μm-slit microbeam irradiation was performed to investigate the possible impacts on the cell-cycle phases via cell–cell contacts. Collectively, we could not detect the intercellular regulation of the cell cycle in HeLa–FUCCI cells, which suggested that the unregulated cell growth was a malignant tumor. Our findings indicated that there was no significant intercellular control system of the cell cycle in malignant tumors during or after radiotherapy, highlighting the differences between normal tissue and tumor characteristics. [ABSTRACT FROM AUTHOR]

Highlights: [•] We report a novel mechanism of radiation protection of DNA by chemical activity of ascorbic acid. [•] The “chemical repair” of DNA damage was revealed using biochemical assay and chemical kinetics analysis. [•] We found that ascorbic acid significantly repairs precursors of nucleobase lesions and abasic sites. [•] However, ascorbic acid seldom repairs precursors of DNA-strand breaks. [ABSTRACT FROM AUTHOR]

Purpose: To develop a method for simulating the dynamics of the photoelectrons and Auger electrons ejected from DNA molecules irradiated with pulsed monochromatic X-rays. Materials and methods: A 30-base-pair (bp) DNA molecule was used as the target model, and the X-rays were assumed to have a Gaussian-shaped time distribution. Photoionization and Auger decay were considered as the atomic processes. The atoms from which the photoelectrons or Auger electrons were emitted were specified in the DNA molecule (or DNA ion) using the Monte Carlo method, and the trajectory of each electron in the electric field formed around the positively charged DNA molecule was calculated with a Newtonian equation. The kinetics of the electrons produced by irradiation with X-rays at an intensity ranging from 1 × 1012 to 1 × 1016 photons/mm2 and energies of 380 eV (below the carbon K-edge), 435 eV (above the nitrogen K-edge), and 560 eV (above the oxygen K-edge) were evaluated. Results: It was found that at an X-ray intensity of 1 × 1014 photons/mm2 or less, all the produced electrons escaped from the target. However, above an X-ray intensity of 1 × 1015 photons/mm2 and an energy of 560 eV, some photoelectrons that were ejected from the oxygen atoms were trapped near the target DNA. Conclusions: A simulation method for studying the trajectories of electrons ejected from a 30-bp DNA molecule irradiated with pulsed monochromatic X-rays has been developed. The present results show that electron dynamics are strongly dependent on the charged density induced in DNA by pulsed X-ray irradiation. [ABSTRACT FROM AUTHOR]

Abstract: Two histidine-containing natural dipeptides, carnosine and anserine (β-alanyl-1-methyl-l-histidine), have been examined for their antioxidant and radioprotective abilities. Pulse radiolysis studies indicated the antioxidative properties of carnosine and anserine aqueous solutions at different pH. The rate constants for the reaction OH radical with carnosine at neutral pH were determined to be 5.3×109 M−1 s−1 at 300nm, and 4.1×109 M−1 s−1 at 400nm, respectively. Carnosine and anserine also protected plasmid pUC18 DNA from X-ray radiation-induced strand breaks as evidenced from the studies by agarose gel electrophoresis. Carnosine showed higher protective efficiency under the experimental conditions. Our data demonstrated that carnosine and anserine may play an important role in the maintenance of the antioxidant system. [Copyright &y& Elsevier]

Silybin (extracted from Silybum marianum) is the major active constituent of silymarin which possesses a wide range of medicinal properties. These properties may be, in part, due to the potent scavenging capacity of oxidizing free radicals. In this context, scavenging radicals (hydroxyl, azide, dibromide anion radicals, nitrite, carbonate, etc.) of silybin have been studied to understand the mechanistic aspects of its action against free radicals. The transients produced in these reactions have been assigned and the rate constants have been measured by pulse radiolysis techniques. Reduction potential determined both by cyclic voltammetry gave a value 0.62±0.02 V vs NHE at pH 9. Quantum chemical calculations have been performed to further confirm the different activities of individual hydroxyl groups with the difference of heat of formation. Moreover, silybin also protected plasmid pUC18 DNA from soft X-ray radiation which induced strand breaks. These results are expected to be helpful for a better understanding of the anti-oxidative properties of silybin. [ABSTRACT FROM AUTHOR]

Purpose: To characterize the complexity of DNA damage through determination of the yields of single (SSB) and double strand breaks (DSB), base lesions and clustered damage sites induced in fully hydrated plasmid DNA by direct radiation effects as a function of the ionizing density of the radiation using 4He2+ ion irradiation with linear energy-transfer (LET) values in the range 19 to 148 keV/μm. Materials and methods: Hydrated plasmid DNA (pUC18) containing 34.5 water molecules/nucleotide was irradiated with He2+ ions with LET values of 19, 63, 95, 121 and 148 keV/μm. From quantification of the conformational changes of the irradiated samples (closed circular, open or linear forms) analyzed by agarose gel electrophoresis, the yields of SSB and DSB were obtained. Base lesions were visualized as additional strand breaks by treatment with base excision repair enzymes (endonuclease III (Nth) and formamidpyrimidine DNA glycosylase (Fpg)). Results: The yield of prompt SSB does not depend significantly on LET of the 4He2+ ions, whereas the yield of prompt DSB increases with increasing LET. The yields of isolated base lesions, revealed by Nth and Fpg as additional SSB, decrease drastically with increasing LET. The sum of the yields of DSB and additional DSB revealed by Nth and Fpg increase with increasing LET of the 4He2+ ions except at the highest LET investigated. Conclusion: The yields of clustered damage, revealed as DSB and non-DSB clustered damage sites, but not isolated lesions, namely SSB, increase with increasing ionization density of the 4He2+ ions except at the highest LET investigated. [ABSTRACT FROM AUTHOR]