3D Characterization of the Molecular Neighborhood of •OH Radical in High Temperature Water by MD Simulation and Voronoi Polyhedra.

Understanding the properties of the •OH radical in aqueous environments is essential for biochemistry, atmospheric chemistry, and the development of green chemistry technologies. In particular, the technological applications involve knowledge of microsolvation of the •OH radical in high temperature water. In this study, the classical molecular dynamics (MD) simulation and the technique based on the construction of Voronoi polyhedra were used to provide 3D characteristics of the molecular vicinity of the aqueous hydroxyl radical (•OHaq). The statistical distribution functions of metric and topological features of solvation shells represented by the constructed Voronoi polyhedra are reported for several thermodynamic states of water, including the pressurized high-temperature liquid and supercritical fluid. Calculations showed a decisive influence of the water density on the geometrical properties of the •OH solvation shell in the sub- and supercritical region: with the decreasing density, the span and asymmetry of the solvation shell increase. We also showed that the 1D analysis based on the oxygen-oxygen radial distribution functions (RDFs) overestimates the solvation number of •OH and insufficiently reflects the influence of transformations in the hydrogen-bonded network of water on the structure of the solvation shell.

Dynamics of Ion Pairing in Dilute Aqueous HCl Solutions by Spectroscopic Measurements of Hydroxyl Radical Conversion into Dichloride Radical Anions.

The rate of formation of dichloride anions (Cl2•-) in dilute aqueous solutions of HCl (2-100 mmol·kg-1) was measured by the technique of pulse radiolysis over the temperature range of 288-373 K. The obtained Arrhenius dependence shows a concentration averaged activation energy of 7.3 ± 1.8 kJ·mol-1, being half of that expected from the mechanism assuming the •OHCl- intermediate and supporting the ionic equilibrium-based mechanism, i.e., the formation of Cl2•- in the reaction of •OH with a hydronium-chloride (Cl-·H3O+) contact ion pair. Assuming diffusion-controlled encounter of the hydronium and chloride ions and including the effect of the ionic atmosphere, we showed that the reciprocal of τ, the lifetime of (Cl-·H3O+), follows an Arrhenius dependence with an activation energy of 23 ± 4 kJ·mol-1, independent of the acid concentration. This result indicates that the contact pair is stabilized by hydrogen bonding interaction of the solvent molecules. We also found that at a fixed temperature, τ is noticeably increased in less-concentrated solutions (mHCl < 0.01 m). Since this concentration effect is particularly pronounced at near ambient temperatures, the increasing pair lifetime may result from the solvent cage effect enhanced by the presence of large supramolecular structures (patches) formed by continuously connected four-bonded water molecules.

Ionic-Equilibrium-Based Mechanism of •OH Conversion to Dichloride Radical Anion in Aqueous Acidic Solutions by Kinetic and Theoretical Studies.

A new mechanism for the dichloride radical anion (Cl2•-) formation in diluted acidic chloride solutions is proposed on the grounds of pulse radiolysis measurements of the optical absorption growth at 340 nm and the density functional theory and Hartree-Fock computations. We show that the rate of •OH conversion into Cl2•- is determined by the equilibrium concentration of the ionic pair H3O+·Cl-. According to the proposed mechanism, the diffusional encounter of •OH and H3O+·Cl- is followed by fast concerted charge/proton transfer ( k(25 °C) = 6.2 × 1012 s-1) to yield Cl•, which then reacts with Cl- to produce Cl2•-. The mechanism has been confirmed by the observed first-order growth of the Cl2•- absorption and a direct proportionality of the rate constant to the activities of H3O+ and Cl- ions. The salt effect on the rate of Cl2•- formation is due to the ionic strength effect on the equilibrium H3O+ + Cl- ⇄ H3O+·Cl-.

Investigation of the bystander effect in CHO-K1 cells.

AIM: Investigation of the bystander effect in Chinese Hamster Ovary cells (CHO-K1) co-cultured with cells irradiated in the dose range of 0.1-4 Gy of high LET 12C ions and X-rays. BACKGROUND: The radiobiological effects of charged heavy particles on a cellular or molecular level are of fundamental importance in the field of biomedical applications, especially in hadron therapy and space radiation biology. MATERIALS AND METHODS: A heavy ion 12C beam from the Heavy Ion Laboratory of the University of Warsaw (HIL) was used to irradiate CHO-K1 cells. Cells were seeded in Petri dishes specially designed for irradiation purposes. Immediately after irradiation, cells were transferred into transwell culture insert dishes to enable co-culture of irradiated and non-irradiated cells. Cells from the membrane and well shared the medium but could not touch each other. To study bystander effects, a clonogenic survival assay was performed. RESULTS: The survival fraction of cells co-cultured with cells irradiated with 12C ions and X-rays was not reduced. CONCLUSIONS: The bystander effect was not observed in these studies.