Cellular exposure to ionizing radiation leads to oxidizing events that alter atomic structure through direct interactions of radiation with target macromolecules or via products of water radiolysis. Further, the oxidative damage may spread from the targeted to neighboring, non-targeted bystander cells through redox-modulated intercellular communication mechanisms. To cope with the induced stress and the changes in the redox environment, organisms elicit transient responses at the molecular, cellular and tissue levels to counteract toxic effects of radiation. Metabolic pathways are induced during and shortly after the exposure. Depending on radiation dose, dose-rate and quality, these protective mechanisms may or may not be sufficient to cope with the stress. When the harmful effects exceed those of homeostatic biochemical processes, induced biological changes persist and may be propagated to progeny cells. Physiological levels of reactive oxygen and nitrogen species play critical roles in many cellular functions. In irradiated cells, levels of these reactive species may be increased due to perturbations in oxidative metabolism and chronic inflammatory responses, thereby contributing to the long-term effects of exposure to ionizing radiation on genomic stability. Here, in addition to immediate biological effects of water radiolysis on DNA damage, we also discuss the role of mitochondria in the delayed outcomes of ionization radiation. Defects in mitochondrial functions lead to accelerated aging and numerous pathological conditions. Different types of radiation vary in their linear energy transfer (LET) properties, and we discuss their effects on various aspects of mitochondrial physiology. These include short and long-term in vitro and in vivo effects on mitochondrial DNA, mitochondrial protein import and metabolic and antioxidant enzymes.

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Multiphoton ionization of neat liquid D2O at room temperature (295 K) with 2-eV subpicosecond laser pulses is used to study the solvation of electrons in this medium. The set of 20 measured kinetic traces covers a wide range of probing wavelengths (450−1450 nm), which allows us to obtain a global picture of the spectral changes that accompany electron hydration. The construction of transient absorption spectra from a proper normalization of the kinetic traces confirms the well-known existence of two absorbing species, one weakly bound absorbing chiefly in the infrared and a strongly bound one whose spectrum at long times is that of the well-characterized hydrated electron. The transient spectra also reveal the occurrence of a stepwise transition between these two species as well as a concomitant continuous blue shift of the strongly bound electron−solvent configuration. A nonlinear fit performed simultaneously on all the data allows the estimation of the characteristic kinetic and spectral parameters of our previously proposed hybrid model of electron solvation when it is applied to D2O. The global fit closely matches the data for the 20 different probing wavelengths investigated. The electrons are found to get trapped in 0.16 ± 0.02 ps, whereas the stepwise transition and the continuous blue shift characteristic times are 0.41 ± 0.02 and 0.51 ± 0.03 ps, respectively. The extent in energy of the monoexponential blue shift of the strongly bound electron spectrum is 0.34 ± 0.02 eV, a value which is very similar to the one that was found for electron solvation in methanol. Finally, it is estimated that about 34% of the electrons get directly trapped into the strongly bound state.

With the help of Monte Carlo simulation techniques, we study the recombination kinetics of hydrated electrons (e−aq) with H3O+ and OH⋅ which results from the photoionization of pure water with femtosecond pulsed lasers. A full description of the simulation procedure is given and various comparisons are made with analytical formulations of the reaction kinetics. Particular attention is given to the reaction of e−aq with H3O+, which is only partially diffusion controlled and which involves a Coulombic interaction with dielectric saturation effects. We find that the probability of reaction per e−aq –H3O+ encounter is small (∼6%) and that the encounter duration can be of the order of a few picoseconds. The competition between the reaction of e−aq with H3O+ and with OH⋅ is analyzed with the simulations and with the independent reaction times method. Both approaches indicate that the e−aq decay is largely dominated by the reaction of e−aq with OH⋅. The effect of neighboring ionization sites on the e−aq decay kinetics is also included in the simulations to account for different possible densities of ionization sites. The initial separation between the reactants is found to be about 1 nm, in agreement with previous determinations. The significance of this last value and the constraints that it puts on the initial kinetic energy of the photoelectrons is discussed.