[en] Complete text of publication follows. The intense absorption spectrum of the hydrated electron, with maximum at 720 nm in room temperature water, shifts strongly to the red as the temperature is raised. In order to use this strong signal for dosimetry and calculation of second order reaction rates, the extinction coefficient must be measured as a function of temperature (and also pressure in supercritical water). Determination of the absolute extinction coefficient of a transient is a difficult undertaking. We have used two pulse radiolysis methods. In the first, we directly observe the kinetics of hydrated electron scavenging by methyl viologen, to form the strongly colored MV⁺ cation. The extinction coefficient of this long-lived radical was carefully measured by electrochemical and temperature cycling many years ago, so the measured ratio of hydrated electron absorption to MV⁺ absorption gives the desired extinction coefficient of (e-)_aq. The MV⁺ spectrum has only been quantitatively measured up to 200 deg C, so the method is not valid at higher temperature. In the second method, transient absorption of hydrated electron is recorded as it is being scavenged by N₂O or SF₆. Direct comparison of the transient absorption with the measured (N₂ or F^-) product yield allows the extinction coefficient to be calculated. While the purpose of our study was to investigate high temperature water, we were astonished to discover that the room temperature hydrated electron extinction coefficient has been incorrectly reported (low) by 10%. We can now demonstrate how this error arose in calibration of pulse radiolysis yields. It has been remarked for decades that the integrated oscillator strength of the hydrated electron is less than unity. Our new measurements rectify this problem for room temperature, and we will present results for high temperature water.

[en] Complete text of publication follows. The involvement of high-valent iron-oxo intermediates in biological, environmental, and industrial processes is of current interest. Fe^VIO₄^2- is relatively stable and is a potential oxidant in 'green' treatment of polluted waters. By contrast, Fe(V) and Fe(IV) are short-lived transients when produced in aqueous solution in the absence of strongly bonding ligands other than hydroxide, a feature that has limited studies of its reactivity. Fe^VO₄^3- and Fe^IVO₄^4- have been suggested to be the intermediates in the oxidation of inorganic and organic compounds by Fe^VIO₄^3-. Fe^VO₄^3- can be generated easily in the presence of excess Fe^VIO₄^2- through the use of reducing carbon-centered radicals produced in pulse radiolysis. The decay of the oxyiron(V) species is dependent on pH, where completely deprotonated Fe^VO₄^3- decays to a longer lived transient (t_1/2 ∼ seconds) via a first-order process. However, as the pH is lowered, oxyiron(V) disappears by second-order kinetics to form ferric ions and hydrogen peroxide. The second order rate constant observed in the disappearance of Fe(V) increases as the pH is lowered and is of the order of 10⁷ M^-1s^-1. Oxyiron(IV) complex with a simple inorganic ligand, P₂O₇^4- in basic medium can be generated from the corresponding parent complex by oxidation with OH/O^- radical in aqueous solutions. The pyrophosphate complex of iron(IV), formed at pH ≥ 10 is short lived (t_1/2 = 100-600 ms). This complex of iron(IV) disappears by a second order process to form a Fe(III) pyrophosphate complex and molecular oxygen. A premix pulse radiolysis was used to measure the reactivity of Fe^VIO₄^2-, Fe^VO₄^3-, and Fe^IVO₄^4- with cyanides, oxysulfur species, and aminopolycarboxylates (APCs) and their radicals. The oxidation rates decrease with increase in pH and are, in the order of Fe^VO₄^3- > Fe^IVO₄^4- > Fe^VIO₄^2-. Reduction rate constants of Fe^VIO₄^2- to Fe^VO₄^3- by ^·CONH₂, ^·SO₃^- and S₄O₆^·3- radicals were found to be 2.6 ± 0.6 x 10⁹, 1.9 ± 0.3 x 10⁸ and 7.5 ± 0.8 x 10⁷ M^-1 s^-1. The oxidation of cyanides by Fe(V) indicate a two-electron transfer with SCN^- while one-electron reduction of Fe^V to Fe^IV to Fe^III takes place in case of CN^-. The formation of a Fe(IV)-cyano complex in reduction of Fe^V by metalcyanide will be discussed. The reaction rates of Fe^VO₄^3- with the two oxysulfur ions were separated by an order of magnitude, with SO₃^2- reacting at 3.9 ± 0.3 x 10⁴ while S₂O₃^2- reacted with Fe(V) at 2.1 ± 0.1 x 10³ M^-1 s^-1 at pH 11.4. Measurements for Fe^VO₄^3- reactivity with oxy-sulfur species suggest two-electron reduction. Fe(V) reactivity with APCs at pH 12.5 increase in order tertiary < secondary < primary amines and proceed via a two-electron oxidation.

[en] Complete text of publication follows. Analysis of the reactions involved in the direct effect of ionizing radiation on DNA is crucial to assessing the risks related with exposure at low dose. The direct interaction of ionizing radiation with DNA initially results in free radicals situated on bases and the backbone, which eventually lead to stable end products that include strand breaks (sb) and free nucleobase release (fbr). The yields of these two products are thought to be related because ejection of an electron from the DNA backbone produces a radical cation that deprotonates to yield a neutral carbon-centered deoxyribose radical. These neutral radicals react when dissolved to produce one strand break and one free base each. Therefore fbr can be used as an indicator of sb. It is commonly presumed that that sb occur independent of the surrounding base context. However recent studies have indicated that a base may indeed have influence over the probability of sb at its backbone unit. In one such study, films prepared from 10- to 30-mer DNA duplexes were irradiated at RT under air using X-rays generated by a tungsten tube operated at 70 kV. The films were dissolved in nuclease free water and stored at 277 K. Unaltered free base release was measured using HPLC, and the yields determined for each base were not strictly proportionate to their presence in the DNA sequence. In fact, this study indicated that strand breaks may be influenced by a number of factors including position within the oligomer as well as the base and its base context. The current study involves further analysis of these factors; instead of using HPLC to separate and measure fbr, which is time consuming and expensive, a novel analytical technique is being used to determine the amount and ratio of fbr for each of the four bases. This technique involves separation of free bases from bulk DNA using filters followed by decomposition of the UV spectra of mixtures of bases at different pH. Decomposition is performed using the Levenberg-Marquardt algorithm for non-linear optimization. To date we have been able to determine relative amounts of nucleobase in mixtures of all four bases with accuracy comparable to HPLC while using much smaller quantities of DNA. Supported by PHS Grant 2-R01-CA32546 of the NCI.

[en] Complete text of publication follows. Bridged bifunctional molecules of general structure X-(CH₂)_n-Y represent an interesting class of models for experimental and theoretical investigations of the early stages of radiation-induced processes in complex organic molecules and macromolecules, including those of biological importance. Variations in ionization energy of functional groups (X and Y), bridge length and conformation may affect crucially the electronic structure and reactivity of primary ionized molecules, which can be described in terms of 'fine tuning'. This contribution gives an overview of our recent studies on a number of bifunctional radical cations (in particular, diketones amidoesters, amidoamines and aminoethers) using EPR, optical spectroscopy and quantum-chemical calculations [1 - 3]. The criterion of localization of spin density in bridged radical cations will be formulated. The effect of 'magic bridge' (or 'conformational lock') on the reactivity of radical cations will be considered in detail. Several examples of selective and specific reactions for radical cations with a 'magic bridge' (n = 3) will be presented. In addition, preliminary data on the properties of bridged bifunctional radical anions will be discussed. This work was supported by the Russian Foundation for Basic Research (project no. 09-03-00848).

[en] Complete text of publication follows. DNA-binding polycations, most notably histones, are known to confer protection to DNA against radiation induced damage. Until recently, DNA protection was believed to be entirely against the indirect effect. The purpose of this study is to test if strong binding between polycations and DNA confers significant radioprotection against the direct effect and to understand how polycations modify the direct-type DNA damage. Our approach is to quantify direct type damage in terms of free radical trapping and unaltered free base release by the DNA. This approach takes advantage of the fact that deoxyribose free radicals are unstable intermediates in the reaction pathways that lead to strand breaks and free base release. Solid state films were prepared from the dodecamer d(CGCGAATTCGCG)2 alone, peptide (KKKKY) alone, and dodecamer-peptide complex. The films were hydrated under a relative humidity of 8%, which was assumed to give Γ ≅ 2.5 mole water/mole nucleotide for the dodecamer alone. Free radical yields were measured by EPR at 4 K for films with DNA-phosphate:lysine ratios of 1:0, 2:1, 1:1, 1:2 and 0:1. We found free radical yields, in nmol/J, of 346 ± 19, 598 ± 8, 793 ± 25, 900 ± 43 and 825 ± 31, respectively. The yields were calculated based on a presumed target mass consisting of the dodecamer, 2.5 waters per DNA-phosphate, Lys binding to up to half of the DNA-phosphates with Cl- associated with the unbound Lys, Na⁺ bound to the remaining DNA-phosphates, and 5.5 waters per Lys. These results suggest that free radical trapping in the dodecamer is enhanced by peptide binding. Experiments are in progress using HPLC to measure base release. The effects of proteins such as histones binding to DNA, are being studied by EPR and HPLC. A comparison of base release with free radical trapping is expected to provide new insights into the mechanisms by which polycation/protein binding influences direct-type DNA damage. Supported by PHS Grant 2-R01-CA32546 of the NCI.

[en] Complete text of publication follows. Ionic liquids may provide a solution to many technological problems. Their properties, negligible vapor pressure, non-flammability and ability to be reused made them attractive alternatives to classical solvents. Our understanding of this new class of solvents is quite poor in some areas. The radiation chemistry of ILs is one of them. Recently absorption spectra of Br₂^·-, (SCN)₂^·- and BrSCN^·- was observed. The rate constants of several elementary reactions of the intermediate formation in the solution of dibromoethane, SCN^- and Br^- in methyltributylammonium bis[(trifluoromethyl)sulfonyl] imide R₄NNTf₂ were studied by ns pulse radiolysis. The rate constants of the reactions between Br₂^·- and SCN^- are of the same order of magnitude as for Br₂^·- with chloropromazine in R₄NNTf₂. The energy of Br^- solvation is the driving force of Br₂^·- formation. At this time the formation of Br₂^·- radical anions with azide anions were studied by pulse radiolysis. Br₂^·- + N₃^- → 2 Br^- + N₃^·. We demonstrated that N₆^·- can be generated in ionic liquids methyltributylammonium bis[(trifluoromethyl)sulfonyl] imide R₄NNtf₂ according to a reaction. N₃^· + N₃^- ↔ N₆^·- It reveals board absorption band in the range of 650 - 700 nm. Alfassi et al. have shown that N₆^·- absorb at 650 nm with extinction coefficient of 8000 M^-1cm^-1.

[en] Complete text of publication follows. There are two main methods for modelling recombination and spin dynamics in radiation chemical systems: a Monte Carlo random flights method (MC), in which the trajectories of the diffusing species are followed explicitly; the Independent Reaction Times method (IRT), in which reaction times are sampled from appropriate marginal distribution functions. This paper reports recent developments to both methods, which extend the range of systems for which they can be used. The main chemical problem with the IRT method is the problem of dealing with secondary reactions of reactive products. The main difficulty is the requirement to generate distances from the new particle to the remaining reactants. Several methods have been proposed in the past, none of which is wholly satisfactory. This paper reports a new 'First Passage Approach' algorithm that allows the new distance to be generated conditional on the IRT that already existed for that pair. This removes a source of bias that is present in the current methods. Tests against full MC simulations show (i) that the new method introduces new particles with the correct spatial distribution and (ii) that it is more accurate than current methods in modelling the subsequent kinetics. Methods for simulating coherent spin evolution in radiation chemical systems have been proposed and implemented before, but it has not been possible to include incoherent spin relaxation in the Hilbert space formalism used. A method for simulating spin relaxation based on discrete events suggested by Brocklehurst has been implemented in both MC and IRT models and tested for a single radical pair against methods using Liouville space and the density matrix formulation. Essentially exact agreement is found, permitting spin models to be generalised with the addition of spin relaxation (both T1 and T2). Spin effects leading to polarisation require the action of distance-dependent interactions such as exchange. It is straightforward to include these in trajectory based MC simulations as the spin evolution becomes an integration along the diffusion path. However, it has always been thought to be impossible to do this using the much faster IRT method. By partitioning the space surrounding each particle into small shells and including the first passage times between these shells it is possible to generalise the IRT method to model these interactions with a comparable accuracy to the MC method, both for neutral and charged pairs. This new method makes it possible to simulate effects such as polarisations with a much better statistical precision than before because of the ability to perform many more realisations.

[en] Complete text of publication follows. Thermo and pH-responsive gels have received much interest in different fields of knowledge for their applications in drug delivery systems, controlled release systems for targeted delivery to specific areas of body and others. Poly(acryloyl-L-proline methyl ester) (A-ProOMe) exhibits a lower critical temperature (LCST) of 14 deg C and polyacrylic acid (PAAc) exhibits a critical pH response at about 4.5 deg C. In this work, three different systems containing (A-ProOMe) and (PAAc) were synthesized by gamma radiation; a) A-ProOMe grafted onto PAAc hydrogel 'comb type hydrogel'; b) radiation binary grafting of A-ProOMe and AAc onto PP by mutual irradiation method PP-g-AAc/A-ProOMe; and c) and binary grafting of those monomers in two step radiation method (PP-g- AAc)-g-(A-ProOMe). The characterization of the graft copolymers was examined by infrared (FTIR-ATR), and thermal analysis (TGA). Limit swelling behavior, pH sensitivity and Cu⁺² uptake of different systems will be discussed. The authors wish to express their thanks to S. Castillo-Rojas, B. Leal, F. Garcia, and M. Cruz, from ICN-UNAM, for technical assistance. This work was supported by DGAPA-UNAM Mexico, Grant IN200108.

[en] Complete text of publication follows. Exciton decay dynamics in semiconductor quantum dots and single-walled carbon nanotubes are extensively studied experimentally. Excitons produced in these nanoobjects decay either by radiative process or by charge transfer to ligands bound to them or by Auger recombination. The number of excitons produced in each nanoobject is usually very small and fluctuations in the number of excitons in each nanoobject are comparable to the average number of excitons per nanoobject. In this situation the conventional bulk approach that considers only the average number of excitons per nanoobject is not sufficient to describe the dynamics of exciton decay. Instead one has to use a stochastic approach which properly takes into account fluctuations in the number of excitons in each nanoobject. In this talk I present a stochastic treatment of exciton decay dynamics in nanoobjects and analyse recent experiments on exciton decay in quantum dots by charge transfer to ligands, exciton decay in quantum dots and nanotubes by Auger recombination, and competition between charge transfer to ligands and Auger recombination in exciton decay in quantum dots. The quantized character of the number of excitons in each nanoobject turns out to be essential to understand exciton decay dynamics.

[en] Complete text of publication follows. Sterically hindered piperidine type amines are used as light stabilizers and precursors of stable nitroxide radicals. The peculiarities of their structure are reflected in the reactivity of corresponding radical cations (RC's). Sterically hindered amine RC's are believed to play the key role in antioxidant action of their parent amines as well as in the photoinitiated reactions of selective ω-monooxidation of aliphatic organic substrates by N-haloamines. In this work, the peculiarities of proton transfer between sterically hindered amines and their RCs have been studied by optically detected EPR method. OD EPR technique detects selectively the particles possessing both spin and electrical charge. The method is sensitive to low concentrations of radiation-generated radical ions in liquids. In comparison with ordinary EPR techniques, these advantages of OD EPR facilitate detection of the processes accompanied by separation of spin and charge, such as RC proton transfer reaction. We have used the following amine compounds, listed in the order of decreasing steric crowding around the transferring proton: di-tert-amylamine (DTA), 2,2,6,6-tetramethylpiperidine (TMP), 1,2,2,6,6-pentamethylpiperidine (PMP), 2,6-dimethylpiperidine, piperidine. Triethylamine (TEA) has been employed as reference compound. TMP RC has proved to undergo incomplete proton transfer yielding dimeric complex radical cation. In contrast, the RC of DTA, an acyclic analogue of TMP, has displayed no changes at our timescale. PMP RC has proved liable to transfer β-proton though the reaction is considerably slower than in TEA. As for piperidine and 2,6- dimethylpiperidine RC's, they seem to decay so fast that they fall out of OD EPR time domain (ns). Thus, given specific degree of steric crowding of the reactive site (as for TMP), steric effect of bulky substituents results in stabilization of the distonic intermediates. The effect is sensitive to local environment of the transferring proton. The technique allowed the first detection and unambiguous identification of the intermediate complex. The financial support of RFBR (Grant No 08-03-00741) is greatly acknowledged.