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A re-examination of our Monte-Carlo modeling of the radiolysis of liquid water by low linear-energy-transfer (LET ~ 0.3 keV [micro][m.sup.-1]) radiation is undertaken herein in an attempt to reconcile the results of our simulation code with recently revised experimental hydrated electron ([e.sup.-.sub.aq]) yield data at early times. The thermalization distance of subexcitation electrons, the recombination cross section of the electrons with their water parent cations prior to thermalization, and the branching ratios of the different competing mechanisms in the dissociative decay of vibrationally excited states of water molecules were taken as adjustable parameters in our simulations. Using a global-fit procedure, we have been unable to find a set of values for those parameters to simultaneously reproduce (i) the revised [e.sup.-.sub.aq] yield of 4.0 [+ or -] 0.2 molecules per 100 eV at "time zero" (that is, a reduction of ~20% over the hitherto accepted value of 4.8 molecules per 100 eV), (ii) the newly measured [e.sup.-.sub.aq] decay kinetic profile from 100 ps to 10 ns, and (iii) the time-dependent yields of the other radiolytic species H*, *OH, [H.sub.2], and [H.sub.2][O.sub.2] (up to ~1 [micro]s). The lowest possible limiting "time-zero" yield of [e.sup.-.sub.aq] that we could in fact obtain, while ensuring an acceptable agreement between all computed and experimental yields, was ~4.4 to 4.5 molecules per 100 eV. Under these conditions, the mean values of the electron thermalization distance and of the geminate electron-cation recombination probability, averaged over the subexcitation electron "entry spectrum," are found to be equal to ~139 [Angstrom] and ~18%, respectively. These values are to be compared with those obtained in our previous simulations of liquid water radiolysis, namely ~88 [Angstrom] and ~5.5%, respectively. Our average electron thermalization distance is also to be compared with the typical size (~64-80 [Angstrom]) of the initial hydrated electron distributions estimated in current deterministic models of "spur" chemistry. Finally, our average probability for geminate electron-cation recombination agrees well with an estimated value of ~15% recently reported in the literature. In conclusion, this work shows that an adaptation of our calculations to a lower hydrated electron yield at early times is possible, but also suggests that the topic is not closed. Further measurements of the [e.sup.-.sub.aq] yields at very short times are needed. Key words: liquid water, radiolysis, electron-cation geminate recombination, electron thermalization distance, hydrated electron ([e.sup.-.sub.aq]), [e.sup.-.sub.aq] decay kinetics, time-dependent molecular and radical yields, Monte-Carlo simulations.

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]

Solvatochromic shift of 4,4′-bpyH in aqueous solutions at elevated temperatures up to supercritical conditions and in various organic solvents with different dielectric constants, is investigated by pulse-radiolysis technique. 4,4′-bpyH shows a stronger solvent–solute interaction in water than in organic solvents, perhaps due to the hydrogen bond between 4,4′-bpyH and water. At 380 °C, local density augmentation, namely ρ local – ρ bulk , in supercritical water becomes 280 kg m −3 ( ρ bulk = 208 kg m −3 ), and the density enhancement factor is 8.9. Density fluctuation maximizes when ρ bulk is around 120 kg m −3 . Density inhomogeneity decreases as temperature rises, but is still remarkable at 400 °C. [ABSTRACT FROM AUTHOR]

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 x 10-7 mol J-1 and by assuming the values of the extinction coefficients of Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed. and OH∙ at 782 nm (ελ=782 nm = 17025 M-1 cm-1) and at 263 nm (ελ=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) x 10-7 mol J-1. [ABSTRACT FROM AUTHOR]

Abstract: In the present work, laser photolysis techniques have been used to study the transient species of silybin (abbreviated as SLB) in ethanol solutions. It has been demonstrated that the exposure of silybin to 248nm excimer laser gives rise to monophotonic ionization with a high quantum yield. The transient species have been identified, and a series of related kinetics parameters have also been obtained. For the sake of comparison, we approached the problem using another way to reduce silybin, i.e. attack of solvated electrons, esol −, produced in pulse radiolysis. The rate constant for reaction of esol − was determined to be 3.8×109 dm3 mol−1 s−1 at 720nm, which is consistent with 4.6×109 dm3 mol−1 s−1, obtained by laser photolysis. [Copyright &y& Elsevier]

The method of pulse radiolysis was used to study radiation-induced transformations of carboxymethylcellulose (CMC) of various degree of substitution (DS) in aqueous solution. The kinetics of the reactions of intermediate products of water radiolysis with this polymer indicated that hydroxyl radicals react very rapidly with the CMC, i.e. the rate constant is about 1×109dm3mol-1s-1 and independent of the DS of the CMC. On the other hand, the hydrated electron can be practically neglected as a precursor for macroradicals formation since its reaction rate constant with CMC is low, i.e. 4.0–5.0×106dm3mol-1s-1. As the result of OH radicals attack on the CMC, radicals on the main-chain, as well as on side groups (carboxymethyl) are formed. Environmental conditions determine the life-time of macroradicals, typically tens of minutes were detected. Most importantly, the pH level of the solution was observed to determine the preferential reaction pathway. Lowering the pH led to increased macroradicals recombination and gel formation. [Copyright &y& Elsevier]

Abstract: Carboxymethyl chitosan (CMCTS) at a highly concentrated aqueous solution forms hydrogel by ionizing irradiation. To study on radiation-induced reaction mechanism of CMCTS in an aqueous solution, CMCTS radicals formed by reactions with OH radical were observed by ESR method. As a result of ESR spectral analysis, CMCTS radicals were identified as radicals on carboxymethyl groups. [Copyright &y& Elsevier]