Deciphering the reaction between a hydrated electron and a hydronium ion at elevated temperatures.

The formation of a H˙ atom in liquid water from the reaction of a hydrated electron with a hydronium cation is a very challenging subject in chemical processes. Here picosecond pulse radiolysis measurements are performed at elevated temperatures, up to 350 °C, of acidic H2O and D2O solutions (up to 0.1 mol L(-1) HClO4) with the aim of investigating several issues related to this reaction. First, the red shift of the solvated electron absorption band in D2O with increasing temperature is found to be affected by the presence of D3O(+) in solution. The modified absorption spectra demonstrate the formation of a transient pair between D3O(+) and solvated electrons at elevated temperature (200-250 °C) when the concentration of D3O(+) is higher than 0.05 mol L(-1). For higher temperatures at 300 and 350 °C when the rate constant is almost diffusion controlled, the pair is no longer observable. Second, the presolvated electron in D2O is not scavenged up to 250 °C in solution containing 0.1 mol L(-1) D3O(+). Third, the decays in the picosecond range obtained under different conditions show that the rate constants are strongly affected by the temperature and by ionic strength. The model with an equation of log(k/k0) = 2A(I)(0.5)/1 + b(I)(0.5) gives a good fit of the experimental results by taking into account the variation of ionic strength, I, and, in particular, by considering the temperature dependence of the constant A.

Chemical repair of base lesions, AP-sites, and strand breaks on plasmid DNA in dilute aqueous solution by ascorbic acid.

We quantified the damage yields produced in plasmid DNA by γ-irradiation in the presence of low concentrations (10-100 µM) of ascorbic acid, which is a major antioxidant in living systems, to clarify whether it chemically repairs radiation damage in DNA. The yield of DNA single strand breaks induced by irradiation was analyzed with agarose gel electrophoresis as conformational changes in closed circular plasmids. Base lesions and abasic sites were also observed as additional conformational changes by treating irradiated samples with glycosylase proteins. By comparing the suppression efficiencies to the induction of each DNA lesion, in addition to scavenging of the OH radicals derived from water radiolysis, it was found that ascorbic acid promotes the chemical repair of precursors of AP-sites and base lesions more effectively than those of single strand breaks. We estimated the efficiency of the chemical repair of each lesion using a kinetic model. Approximately 50-60% of base lesions and AP-sites were repaired by 10 µM ascorbic acid, although strand breaks were largely unrepaired by ascorbic acid at low concentrations. The methods in this study will provide a route to understanding the mechanistic aspects of antioxidant activity in living systems.

On the spur lifetime and its temperature dependence in the low linear energy transfer radiolysis of water.

In the spirit of the radiation chemical "spur model", the lifetime of a spur (τ(s)) is an important indicator of overlapping spurs and the establishment of homogeneity in the distribution of reactive species created by the action of low linear energy transfer (LET) radiation (such as fast electrons or γ irradiation). In fact, τ(s) gives the time required for the changeover from nonhomogeneous spur kinetics to homogeneous kinetics in the bulk solution, thus defining the so-called primary (or "escape") radical and molecular yields of radiolysis, which are obviously basic to the quantitative understanding of any irradiated chemical system. In this work, τ(s) and its temperature dependence have been determined for the low-LET radiolysis of deaerated 0.4 M aqueous solutions of H(2)SO(4) and pure liquid water up to 350 °C using a simple model of energy deposition initially in spurs, followed by random diffusion of the species of the spur during track expansion until spur overlap is complete. Unlike our previous τ(s) calculations, based on irradiated Fricke dosimeter simulations, the current model is free from any effects due to the presence of oxygen or the use of scavengers. In acidic solutions, the spur lifetime values thus obtained are in very good agreement with our previous calculations (after making appropriate corrections, however, to account for the possibility of competition between oxygen and Fe(2+) ions for H˙ atoms in the Fricke dosimeter, an effect which was not included in our original simulations). In this way, we confirm the validity of our previous approach. As expected, in the case of pure, oxygen-free water, our calculated times required to reach complete spur overlap are essentially the same (within uncertainty limits) as those found in acidic solutions. This explicitly reflects the fact that the diffusion coefficients for the hydrated electron and the H˙ atom that are involved in the overall calculation of the lifetime of spurs in neutral or acidic media, respectively, are of similar magnitude over the 25-350 °C temperature range studied.

Time-dependent yield of the hydrated electron in subcritical and supercritical water studied by ultrafast pulse radiolysis and Monte-Carlo simulation.

Fast kinetics and time-dependent yields of the hydrated electron (e(-)(aq)) in pure water under conditions of high temperature and pressure up to the supercritical region were investigated by picosecond and nanosecond pulse radiolysis experiments. More significant decays at short times followed by plateau components at longer times were observed with increasing temperature, suggesting faster spur reaction processes. In supercritical water, it was also found that the e(-)(aq) yields strongly depend on the pressure (density). Comparison of these measurements with Monte-Carlo computer simulations allowed us to identify spur reactions of e(-)(aq) that occur predominantly at high temperatures and also to provide new key information on certain spur model parameters. In particular, the experimental time-dependent e(-)(aq) yields were best reproduced if the electron thermalization distance decreases with increasing temperature. This "shrinkage" of spur sizes at high temperatures was attributed to an increase in the scattering cross sections of subexcitation electrons, likely originating from a decrease in the degree of structural order of water molecules as the temperature is increased.

Spin-trapping reactions of a novel gauchetype radical trapper G-CYPMPO.

Chemical reactions of a novel gauchetype spin trap, G-CYPMPO (sc-5-(5,5-dimethyl-2-oxo-1,3,2-dioxaphosphinan-2-yl)-5-methy-1-pyrroline N-oxide, O1-P1-C6-N1 torsion angle = 52.8°), with reactive oxygen species were examined by pulse radiolysis technique with 35 MeV electron beam and by electron spin resonance spectroscopy after (60)Co γ-ray irradiation. The spin-trapping reaction rate constants of G-CYPMPO toward the hydroxyl radical and the hydrated electron were estimated to be (4.2 ± 0.1) × 10(9) and (11.8 ± 0.2) × 10(9) M(-1)s(-1), respectively. Half-lives of the spin adducts, hydroxyl radical, and perhydroxyl radical adducted G-CYPMPO were estimated to be ∼35 and ∼90 min, respectively. A comparison of the results with earlier reports using different radical sources suggests that the purity of the solution and/or the radical generation technique may influence the stability of the spin adducts.

Temperature dependence of the Fricke dosimeter and spur expansion time in the low-LET high-temperature radiolysis of water up to 350 °C: a Monte-Carlo simulation study.

Monte-Carlo simulations of the radiolysis of the ferrous sulfate (Fricke) dosimeter with low-linear energy transfer (LET) radiation (such as (60)Co γ-rays or fast electrons) have been performed as a function of temperature from 25 to 350 °C. The predicted yields of Fe(2+) oxidation are found to increase with increasing temperature up to ∼100-150 °C, and then tend to remain essentially constant at higher temperatures, in very good agreement with experiment. By using a simple method based on the direct application of the stoichiometric relationship that exists between the ferric ion yields so obtained G(Fe(3+)) and the sum {3 [g(e(-)(aq) + H˙) + g(HO(2)˙)] + g(˙OH) + 2 g(H(2)O(2))}, where g(e(-)(aq) + H˙), g(HO(2)˙), g(˙OH), and g(H(2)O(2)) are the primary radical and molecular yields of the radiolysis of deaerated 0.4 M H(2)SO(4) aqueous solutions, the lifetime (τ(s)) of the spur and its temperature dependence have been determined. In the spirit of the spur model, τ(s) is an important indicator for overlapping spurs, giving the time required for the changeover from nonhomogeneous spur kinetics to homogeneous kinetics in the bulk solution. The calculations show that τ(s) decreases by about an order of magnitude over the 25-350 °C temperature range, going from ∼4.2 × 10(-7) s at 25 °C to ∼5.7 × 10(-8) s at 350 °C. This decrease in τ(s) with increasing temperature mainly originates from the quicker diffusion of the individual species involved. Moreover, the observed dependence of G(Fe(3+)) on temperature largely reflects the influence of temperature upon the primary free-radical product yields of the radiolysis, especially the yield of H˙ atoms. Above ∼200-250 °C, the more and more pronounced intervention of the reaction of H˙ atoms with water also contributes to the variation of G(Fe(3+)), which may decrease or increase slightly, depending on the choice made for the rate constant of this reaction. All calculations reported herein use the radiolysis database of Elliot (Atomic Energy of Canada Limited) and Bartels (University of Notre Dame) that contains all the best currently available information on the rate constants, reaction mechanisms, and g-values in the range 20 to 350 °C.

Time-dependent radiolytic yield of OH• radical studied by picosecond pulse radiolysis.

Picosecond pulse radiolysis measurements using a pulse-probe method are performed to measure directly the time-dependent radiolytic yield of the OH(•) radical in pure water. The time-dependent absorbance of OH(•) radical at 263 nm is deduced from the observed signal by subtracting the contribution of the hydrated electron and that of the irradiated empty fused silica cell which presents also a transient absoption. The time-dependent radiolytic yield of OH(•) is obtained by assuming the yield of the hydrated electron at 20 ps equal to 4.2 × 10(-7) mol J(-1) and by assuming the values of the extinction coefficients of e(aq)(-) and OH(•) at 782 nm (ε(λ=782 nm) = 17025 M(-1) cm(-1)) and at 263 nm (ε(λ=263 nm) = 460 M(-1) cm(-1)), respectively. The value of the yield of OH(•) radical at 10 ps is found to be (4.80 ± 0.12) × 10(-7) mol J(-1).

Temperature dependent absorption spectra of Br(-), Br2(•-), and Br3(-) in aqueous solutions.

The absorption spectra of Br(2)(•-) and Br(3)(-) in aqueous solutions are investigated by pulse radiolysis techniques from room temperature to 380 and 350 °C, respectively. Br(2)(•-) can be observed even in supercritical conditions, showing that this species could be used as a probe in pulse radiolysis at high temperature and even under supercritical conditions. The weak temperature effect on the absorption spectra of Br(2)(•-) and Br(3)(-) is because, in these two systems, the transition occurs between two valence states; for example, for Br(2)(-) we have (2)Σ(u) → (2)Σ(g) transition. These valence transitions involve no diffuse final state. However, the absorption band of Br(-) undergoes an important red shift to longer wavelengths. We performed classical dynamics of hydrated Br(-) system at 20 and 300 °C under pressure of 25 MPa. The radial distribution functions (rdf's) show that the strong temperature increase (from 20 to 300 °C) does not change the radius of the solvent first shell. On the other hand, it shifts dramatically (by 1 Å) the second maximum of the Br-O rdf and introduces much disorder. This shows that the first water shell is strongly bound to the anion whatever the temperature. The first two water shells form a cavity of a roughly spherical shape around the anion. By TDDFT method, we calculated the absorption spectra of hydrated Br(-) at two temperatures and we compared the results with the experimental data.

Protective effects of silybin and analogues against X-ray radiation-induced damage.

Silybin (SLB) and similar analogues, namely, hesperetin (HESP), naringenin (NAN) and naringin (NAR), are believed to be active constituents of natural flavonoids that have been reported as chemopreventive agents for certain cancers. Moreover, SLB and analogues have been determined to fast repair DNA bases from oxidative damage by pulse radiolysis techniques. The present study was designed to evaluate the protective effects of SLB and analogues on soft X-ray-induced damage to plasmid DNA in vitro. The DNA damage was determined by agarose gel electrophoresis. SLB and analogues were found to protect DNA from radiation damage at micromolar concentrations. Among the compounds tested, HESP and SLB were the most effective in preventing X-ray-induced formation of DNA single-strand breaks (SSB). A comparison of these results with other experiments showed that the ability of SLB and analogues to inhibit DNA damage in vitro correlated with the ability of the compounds to scavenge free radicals. Our work revealed that natural flavonoids, SLB and analogues may be used as potent radioprotectors against radiation damage.

Pulse radiolysis studies on the temperature-dependent spectrum and the time-dependent yield of solvated electron in propane-1,2,3-triol.

With a revisit of the absorption coefficient of the solvated electron in propane-1,2,3-triol, the temperature-dependent behavior of the absorption spectrum of solvated electron was studied from room temperature to 573 K by pulse radiolysis techniques. The change in the absorption spectrum of solvated electron in propane-1,2,3-triol observed by cooling down from a high temperature to 333 K is compared with that occurring during the electron solvation process at 333 K. The effect of the specific molecular structure of propane-1,2,3-triol compared to other alcohols is discussed.

Water radiolysis with heavy ions of energies up to 28 GeV. 3. Measurement of G(MV*+) in deaerated methyl viologen solutions containing various concentrations of sodium formate and Monte Carlo simulation.

Formation yields of methyl viologen cation radicals G(MV*+) (100 eV)(-1) have been measured in deaerated aqueous solutions of 0.25 mM methyl viologen (MV(2+)) containing various concentrations of formate anion (0.01-2 M) after irradiation with six different ion beams (4He(2+), 12C(6+), 20Ne(10+), 28Si(14+), 40Ar(18+) and 56Fe(26+) with incident energies varying from 0.6 to 28 GeV) provided by the Heavy Ion Medical Accelerator in Chiba (HIMAC) at the National Institute of Radiological Science (NIRS) in Japan. The sample solutions are irradiated at the incident energy of the ions using 1-cm irradiation cells. Corresponding LET values cover the range from 2.2 to 185 eV/nm. G(MV*+) increases with increasing formate concentration. In 4He(2+) radiolysis, it increases from 5.7 to 7.1 as the concentration of formate is increased from 0.01 to 2 M, while in 56Fe(26+) radiolysis, the MV*+ yield value changes from 2.2 to 4.1. The other values lie between the yields for 4He(2+) and 56Fe(26+). In addition, G(MV*+) decreases with increasing LET. In the case of 12C(6+) radiolysis, G(MV*+) increases with increasing energy of the carbon ions from 135 to 400 MeV/nucleon, i.e., with decreasing LET from 21 to 11 eV/nm. In parallel to the above measurements, Monte Carlo simulations of the radiolysis of the MV(2+)/formate solutions have been performed. Ionic strength effects on reactions between charged species are taken into account. To reproduce the experimental results, previously unreported reactions such as e(aq)(-) + MV*+, MV*(+) + *OH and *COO- + *OH have been introduced in the reaction scheme. After optimization, the rate constants of these latter two reactions are determined to be (3 +/- 0.5) x 10(10) and (5 +/- 0.5) x 10(10) M(-1) s(-1), respectively. By contrast, the reaction between e(aq)- and MV*+ is too slow to affect G(MV*+). On the basis of these calculations, characteristics of intratrack reactions induced by heavy-ion beams are discussed in reference not only to the scavenger method used for measurement of water decomposition product yields but also to the differences in the relative spatial distribution of the reactants as well as the places where their intratrack reaction occurs within the geometry of the ion track structure.

Effect of water density on the absorption maximum of hydrated electrons in sub- and supercritical water up to 400 degrees C.

The optical absorption spectra of the hydrated electron (e(aq) (-)) in supercritical (heavy) water (SCW) are measured by electron pulse radiolysis techniques as a function of water density at three temperatures of 380, 390, and 400 degrees C, and over the density range of approximately 0.2-0.65 g/cm(3). In agreement with previous work, the position of the e(aq) (-) absorption maximum (E(A(max) )) is found to shift slightly to lower energies (spectral "redshift") with decreasing density. A comparison of the present E(A(max) )-density data with other measurements already reported in the literature in subcritical (350 degrees C) and supercritical (375 degrees C) water reveals that at a fixed pressure, E(A(max) ) decreases monotonically with increasing temperature in passing through the phase transition at t(c). By contrast, at constant density, E(A(max) ) exhibits a minimum as the water passes above the critical point into SCW. These behaviors are explained in terms of simple microscopic arguments based on the crucial role played by local density and configurational fluctuations (associated with criticality) in providing pre-existing polymeric clusters, which act as trapping sites for electrons.

Fast repair activities towards dGMP hydroxyl radical adducts by silybin and its analogues.

The repair activities of silybin (SLB) and its analogues towards the oxidizing deoxyguanosine monophosphate (dGMP) hydroxyl radical adducts are investigated by pulse radiolytic techniques. On pulse irradiation of nitrous oxide saturated 2.0 mM dGMP aqueous solution containing 0.1 mM silybin at neutral pH, the transient absorption spectrum of the dGMP hydroxyl radical adducts decreases with the formation of the phenoxyl radical of silybin within tens of microseconds, indicating that there is a repair reaction between the dGMP hydroxyl radical adduct and silybin. The rate constant of the repair reaction is calculated to be 1.0 x 10(9) M(-1)s(-1) for silybin. The repair activity of hesperetin (HESP), naringin (NAN) and naringenin (NAR) towards hydroxyl radical adducts of dGMP are also studied.

Electron photodetachment from iodide in ionic liquids through charge-transfer-to-solvent band excitation.

Solvation of iodide and electrons in an ionic liquid (N,N,N-trimethyl-n-propylammonium bis(trifluoromethanesulfonyl)imide; TMPA-TFSI) was studied through the absorption spectra of the charge-transfer-to-solvent (CTTS) state of iodide and of solvated electrons. The interaction between the TMPA cation and iodide was strong, whereas electrons were weakly solvated in TMPA-TFSI. We followed electron photodetachment from iodide to the ionic liquid and formation of the solvated electrons by observing absorption in the visible and near-infrared regions using a nanosecond laser flash photolysis method. The quantum yield of the photodetachment in TMPA-TFSI was estimated to be 0.34, which is much higher than that in a high-concentration aqueous salt solution previously reported. We also examined a reaction of the solvated electrons with the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (Bmim-TFSI) as a solute in TMPA-TFSI. The reaction rate was determined to be 5.3 x 10(8) M(-1) s(-1). The electrons before full solvation (dry electrons) reacted with Bmim cations efficiently. These observations suggest that the electrons in TMPA-TFSI can move easily before solvation.

Reaction between diiodide anion radicals in ionic liquids.

Photodetachment of electrons from iodide ions produced diiodide anion radicals in ionic liquids containing ammonium, pyrrolidinium, and piperidinium cations. The rates of reaction between diiodide anion radicals in molecular solvents such as H2O, methanol, and ethanol could be estimated by the Debye-Smoluchowski equation, which accounts for electrostatic interactions using dielectric constants for the molecular solvents. In contrast, the rates of reaction between diiodide anion radicals in the ionic liquids were close to the diffusion-limited rates for the neutral molecules, suggesting that electrostatic repulsion between the diiodide anion radicals is weakened by Coulombic shielding in the ionic liquids.

Free radical scavenger edaravone suppresses x-ray-induced apoptosis through p53 inhibition in MOLT-4 cells.

Edaravone, a clinical drug used widely for the treatment of acute cerebral infarction, is reported to scavenge free radicals. In the present study, we investigated the radioprotective effect of edaravone on X-ray-induced apoptosis in MOLT-4 cells. Apoptosis was determined by the dye exclusion test, Annexin V binding assay, cleavage of caspase, and DNA fragmentation. We found that edaravone significantly suppressed the X-ray-induced apoptosis. The amount of intracellular ROS production was determined by the chloromethyl-2',7'-dichlorodihydro-fluorescein diacetate system. We found that the intracellular ROS production by X-irradiation was completely suppressed by the addition of edaravone. The accumulation and phosphorylation of p53 and the expression of p21(WAF1), a target protein of p53, which were induced by X-irradiation, were also suppressed by adding edaravone. We conclude that the free radical scavenger edaravone suppresses X-ray-induced apoptosis in MOLT-4 cells by inhibiting p53.

Pulse radiolysis study on free radical scavenger edaravone (3-methyl-1-phenyl-2-pyrazolin-5-one).

The reactions between edaravone and various one-electron oxidants such as (*)OH, N(3)(*), Br(2)(-), and SO(4)(-), have been studied by pulse radiolysis techniques. The transient species produced by the reaction of edaravone with (*)OH radical shows an absorption band with lambda(max)=320 nm, while the oxidation by N(3)(*), Br(2)(-), SO(4)(-) and CCl(3)OO(*) results in an absorption band with lambda(max)=345 nm. Different from the previous reports, the main transient species by the reaction of edaravone with (*)OH radical in the absence of O(2) is attributed to OH-adducts. At neutral condition (pH 7), the rate constants of edaravone reacting with (*)OH, N(3)(*), SO(4)(-), CCl(3)OO(*), and e(aq)(-) are estimated to be 8.5x10(9), 5.8x10(9), 6x10(8), 5.0x10(8) and 2.4x10(9)dm(3)mol(-1)s(-1), respectively. From the pH dependence on the formation of electron adducts and on the rate constant of edaravone with hydrated electron, the pK(a) of edaravone is estimated to be 6.9+/-0.1.

Application of accelerators for the research and development of scintillators.

We introduce experimental systems which use accelerators to evaluate scintillation properties such as scintillation intensity, wavelength, and lifetime. A single crystal of good optical quality is often unavailable during early stages in the research and development (R&D) of new scintillator materials. Because of their beams' high excitation power and/or low penetration depth, accelerators facilitate estimation of the properties of early samples which may only be available as powders, thin films, and very small crystals. We constructed a scintillation spectrum measurement system that uses a Van de Graaff accelerator and an optical multichannel analyzer to estimate the relative scintillation intensity. In addition, we constructed a scintillation time profile measurement system that uses an electron linear accelerator and a femtosecond streak camera or a microchannel plate photomultiplier tube followed by a digital oscilloscope to determine the scintillation lifetimes. The time resolution is approximately 10 ps. The scintillation spectra or time profiles can be obtained in a significantly shorter acquisition time in comparison with that required by conventional measuring systems. The advantages of the systems described in this study can significantly promote the R&D of novel scintillator materials.

High-LET ion radiolysis of water: visualization of the formation and evolution of ion tracks and relevance to the radiation-induced bystander effect.

Ionizing radiation-induced bystander effects, commonly observed in cell populations exposed to high-linear energy transfer (LET) radiations, are initiated by damage to a cellular molecule which then gives rise to a toxic signal exported to neighboring cells not directly hit by radiation. A major goal in studies of this phenomenon is the identification of this initial radiation-induced lesion. Liquid water being the main constituent of biological matter, reactive species produced by water radiolysis in the cellular environment are likely to be major contributors to the induction of this lesion. In this context, the radiation track structure is of crucial importance in specifying the precise location and identity of all the radiolytic species and their subsequent signaling or damaging effects. We report here Monte Carlo track structure simulations of the radiolysis of liquid water by four different impacting ions 1H+, 4He2+, 12C6+ and 20Ne10+, with the same LET ( approximately 70 keV/ microm). The initial radial distribution profiles of the various water decomposition products (eaq(-), *OH, H*, H2 and H2O2) for the different ions considered are presented and discussed briefly in the context of track structure theory. As an example, the formation and temporal evolution of simulated 24 MeV 4He2+ ion tracks (LET approximately 26 keV/microm) are reported for each radiolytic species from 1 ps to 10 micros. The calculations reveal that the ion track structure is completely lost by approximately 1 micros.

Contribution of indirect action to radiation-induced mammalian cell inactivation: dependence on photon energy and heavy-ion LET.

The contribution of indirect action mediated by OH radicals to cell inactivation by ionizing radiations was evaluated for photons over the energy range from 12.4 keV to 1.25 MeV and for heavy ions over the linear energy transfer (LET) range from 20 keV/microm to 440 keV/microm by applying competition kinetics analysis using the OH radical scavenger DMSO. The maximum level of protection provided by DMSO (the protectable fraction) decreased with decreasing photon energy down to 63% at 12.4 keV. For heavy ions, a protectable fraction of 65% was found for an LET of around 200 keV/microm; above that LET, the value stayed the same. The reaction rate of OH radicals with intracellular molecules responsible for cell inactivation was nearly constant for photon inactivation, while for the heavy ions, the rate increased with increasing LET, suggesting a reaction with the densely produced OH radicals by high-LET ions. Using the protectable fraction, the cell killing was separated into two components, one due to indirect action and the other due to direct action. The inactivation efficiency for indirect action was greater than that for direct action over the photon energy range and the ion LET range tested. A significant contribution of direct action was also found for the increased RBE in the low photon energy region.