Janusz Bednarek

Unexpected radical generation on γ-irradiating metastable forms of water at 77 K. Invited Lecture

The effects of γ-irradiating three metastable forms of water at 77 K have been studied by electron spin resonance spectroscopy and are compared with those of hexagonal ice. Two amorphous forms of water were made by so-called hyperquenching of liquid water droplets (hyperquenched glassy water, HGW) and by deposition of water vapours (amorphous solid water, ASW) at 77 K. Metastable cubic ice was made either by heating HGW, or by hyperquenching liquid water droplets at 170 K. Whereas γ-irradiation of hexagonal ice at 77 K produces only OH radicals as primary radiation product, on γ-irradiation of HGW and ASW comparable yields of OH and HO2 radicals were observed, and their ratio did not depend on radiation dose. Mainly OH radicals were formed on γ-irradiating cubic ice made directly from liquid water. In contrast, comparable amounts of OH and HO2 radicals were formed on irradiation of cubic ice made by heating HGW, and their relative yields depend strongly on the irradiation dose. HO2 radicals are formed as primary products of radiation, as are OH radicals, and their formation is attributed to the abundant presence of defects, with two oxygens facing each other without hydrogen in between, in HGW, in ASW and in disordered regions of cubic ice. When small amounts of thymine were added to HGW, thymine radicals were formed by addition of atomic hydrogen on γ-irradiation already at 77 K at the expense of the relative contribution of HO2 radicals. This is not due to scavenging of once-formed HO2 radicals by thymine, but it occurs concurrently with HO2 radical formation. Therefore, it is rationalized by a concerted process involving the primary intermediate H2O+. The radiation effect on thymine must be indirect because of the large water-to-thymine ratio of ≈104 in homogeneous glassy solution. Possible implications of these studies are discussed for our understanding of (i) defects in metastable forms of water and their detection via HO2 radical formation, (ii) radiation damage to biomolecules in aqueous solution, and (iii) radical generation on high-energy irradiation of ASW in outer space and the secondary formation of stable molecules on decay of the radicals.

Single-crystal electron spin resonance studies on radiation-produced species in ice Ih. Part 2.—The HO2 radicals

The X-band e.s.r. spectra were recorded at 4 K for single crystals of hexagonal D2O ice gamma-irradiated at 77 K. From the measurements at 4 K the g tensors as well as the direction cosines were determined for O– radicals formed from OD radicals upon cooling. There are two sets of O– radicals derivable directly from three sets of OD radicals trapped in ice Ih. The sp2 hybridization of valence orbitals for O– radicals is consistent with anisotropic g tensors.

Reaction dynamics in glass transition region: Propagating radicals in ultraviolet‐irradiated poly(methyl methacrylate)

The decay kinetics of propagating radicals in UV‐irradiated poly(methyl methacrylate) (PMMA) was followed at the irradiation temperatures in the range 300–400 K. At the highest temperatures, in the glass transition region of PMMA the radical decay patterns approached that of classical kinetics. Below the glass transition region, increase of reaction dispersivity with temperature decrease was found to be accompanied by a marked decrease, of about two orders of magnitude, of activation energy for the reaction. Both these phenomena are rationalized in terms of the stochastic model [A. Plonka and A. Paszkiewicz, J. Chem. Phys. 96, 1128 (1992)], which includes structural reorganization of the host matrix by imposing upon the static disorder model for radical reaction the additional assumption that matrix relaxation randomly reassigns the reactivity of guest species. Effective relaxation is shown to lead to the classical pattern of radical decay and the accompanying marked increase of activation energy is seen to be due to the contribution of activation energy for matrix structural relaxation.

Electron spin resonance studies on tryptophan photolysis in frozen micellar systems of anionic surfactants

Abstract UV photolysis of tryptophan (Trp) was studied by the electron spin resonance method in frozen solutions of simple micelles (SMs) and reversed micelles (RMs) of anionic surfactants: sodium bis(2-ethylhexyl)sulphosuccinate, sodium dodecylsulphate and ammonium perfluoropolyether carbonate. The primary species stabilized at 77 K in protiated systems are Trp + , Trp and alkyl-type radicals while, in the perfluorinated RM system, only Trp + is observed. The yield of paramagnetic species is an order of magnitude higher in microheterogeneous systems than in homogeneous aqueous solutions. It is higher in RM than in SM samples, parallel to the fraction of Trp molecules bound to micellar interface. In the presence of oxygen the primary photoproducts transform into peroxy radicals on heating the sample above 100 K. In deaerated samples, Trp + survives up to temperatures of 190 K or higher (depending on the system).

New model system in radiation cryochemistry:: hyperquenched glassy water

Abstract Radicals generated by high-energy irradiation of liquid water, short-lived at ambient temperature, can be studied at cryogenic temperatures after irradiating water and dilute aqueous solutions in their glassy states which can be obtained by so-called hyperquenching of the liquids at cooling rates of ∼10 6 –10 7 K s −1 . In the glassy states of hyperquenched dilute aqueous solutions there is no problem with phase separation and radiolysis of glassy water is quite distinct from radiolysis of polycrystalline ice obtained from liquid water on slow-cooling in liquid nitrogen.

ESR studies on reactivity of HO2 radicals in polycrystalline ice: Non-monotonic changes of disproportionation rate in the temperature range 100–200 K

Abstract The kinetics of HO2 radicals in UV-irradiated frozen dilute solutions of H2O2 were followed by the ESR technique. Radical disproportionations at the irradiation temperature were found to be adequately described in terms of dispersive kinetics by a second-order equal-concentration kinetic equation with rate parameters changing non-monotonically in the temperature range 100–200 K. These changes are thought to occur due to well separated structural relaxation regions of polycrystalline ice in the above temperature range.

Radiation cryochemistry of frozen dilute aqueous solutions: influence of the extent of solute segregation on the radiolysis pathway

Polycrystalline hexagonal ice containing thymine was γ-irradiated at 77 K, and X-band electron paramagnetic resonance spectra of the resulting radicals were recorded at this temperature immediately after irradiation, and after thermal annealing of the samples at temperatures up to 250 K. The examined frozen samples were obtained from hyperquenched glassy aqueous solution containing 0.005 M of thymine, by using two different procedures: (i) crystallization of the glass by heating up to 250 K, and (ii) warming the glass up to room temperature to melt it, and subsequent cooling of the solution in liquid N2. Thymine-derived radicals were detectable only in the samples obtained by crystallization of the glass. We conclude that the extent of solute segregation is less for the frozen aqueous solution made by route (i) than by route (ii). Therefore, the solute is more hydrated in the frozen sample made by route (i) and the primary products of water radiolysis can react with the solute. The present results are compared with those reported for the thymine/hyperquenched glassy water system (Bednarek et al. , J. Phys. Chem. B 103 , 6824 (1999)).

Low-temperature ESR studies on the photolysis of nitrosyl haemoglobin in aqueous solutions

Abstract Nitrosyl haemoglobin in aqueous solutions was used for low-temperature electron spin resonance studies on the kinetics of photoinduced NO ligand dissociation from the haem group. It was found that fractal time kinetic equations provide an adequate description of the photodissociation reaction over all time and temperature ranges accessible to electron spin resonance measurements.

ESR Evidence of Peroxy Radical Formation during Radiolysis of Frozen Carbohydrate Solutions

Abstract During thermal annealing of the frozen polycrystalline samples of aqueous carbohydrate (dextran, melibiose, maltose, glucose, and xylose) solutions y-irradiated at 77 K the formation of peroxy radicals by addition of oxygen to the primary carbon-based radicals was observed at 190-220 K by the ESR method.