Ionizing radiation, indirect effects

Chemical and biological effects of ionizing radiation are thought to occur through two main mechanisms direct interaction of the radiation with food components and living cells in materials exposed to it, and indirect action from radiolytic products, such as the radicals formed from water molecules (see Chap. 12). 

Indirect effects in nonaqueous media have received very little attention. It is likely that semiselective chemical reactions will occur in organic solvents containing active solutes. Liquid ammonia and sulfur dioxide also might offer some possibilities as solvents. The direct action of densely ionizing radiation on ammonia to produce hydrazine might yield reasonable quantities of this important reagent. 

Ionizing radiation can act in two distinct ways on organic substances. In the absence of water, in condensed systems or in concentrated solution, the predominant effects occur directly on the organic molecule and produce electronic excitations or ionizations which may lead to chemical modification. In dilute solution (1% or less) the major effects are the result of reactions between the solute and reactive species produced by the radiolysis of water. These indirect effects are the subject of this article. 

In the interaction of ionizing radiation with DNA one distinguishes between the direct effect (absorption of the ionizing radiation by DNA) and the indirect... 

Ward RL (1980) Mechanisms of poliovirus inactivation by the direct and indirect effects of ionizing radiation. Radiat Res 83 330-344... 

The experiments can be carried out in solution or in solid state (for example in KBr pellet) in order to study both direct and indirect effects of ionizing radiations. The experiments can also be carried out with samples in crystal, layer or gel. The experiments require only limited amounts of sample, a condition which is often restrictive to study the biological molecules for instance. The size of the studied molecules is not limited small molecules as well as macromolecules, such as the nucleic acids or polymers, can be studied, allowing the identification of radiation-induced effects. 

The guanine moiety has the lowest ionization potential of any of the DNA bases or of the sugar-phosphate backbone. As a result, radiation-produced holes are stabilized as dG for hydrated DNA irradiated at 77 K There is an extensive literature describing the role of dG in the radiation chemistry of DNA as studied by pulse radiolysis, flash photolysis, and product analysis. In order to explicate the oxidative reaction sequence in irradiated DNA and to more firmly identify the relevant radical intermediates, ESR spectroscopy was employed to investigate y-irradiated hydrated DNA (T = 12 2). Some experiments were also performed on hydrated (T = 12 2) DNA in which an electron scavenger [thallium(ni) (TP )] was employed to isolate the oxidative path. Oxygen-17 isotopically enriched water was also used to confirm a proposed water addition step to G and the subsequent transformations that follow These experiments were run in oxygen-free samples under conditions for which indirect effects were unimportant. 

In the interaction of ionizing radiation with cellular DNA, two effects contribute the direct effect [reactions (I) and (2)], where the energy of the ionizing radiation is absorbed by DNA itself and the indirect effect, where the water that surrounds the DNA absorbs the energy of the ionizing radiation [reactions (3) and (4)]. 

OH + e (aq) + n + HzO + H + H2O2 + H2 The first 14 water molecules per nucleotide in the hydration layer surrounding DNA have approximately the same mass as DNA [84] and, therefore, the same number of ionizations are expected to occur in the primary hydration layer as in the DNA strand. However, it is unknown how the water molecules in the primary hydration layer are affected by radiation. One possibility is that water cations and electrons are formed, which transfer their ionic character to the DNA strand (quasi-direct effects). Water cations can also transfer protons to neighboring water molecules resulting in hydroxyl radicals. The products formed in the hydration layer (hydroxyl radicals, hydrogen atoms or aqueous electrons) can subsequently react with DNA (indirect effects). Quasi-direct and indirect effects are expected to yield very different radicals. 

The effect of ionizing radiation on phenols has been stndied mainly in aqneons soln-tions nnder oxidizing conditions, where the phenols are reacted with hydroxyl radicals or with transient one-electron oxidants to yield, indirectly or directly, phenoxyl radicals. The reactions leading to formation of phenoxyl radicals, as well as the properties and reactions of phenoxyl radicals in aqneons solntions, are discnssed in the chapter on transient phenoxyl radicals. In this chapter, other aspects of the radiation chemistry of phenols are summarized. These include studies with phenols in organic solvents and in the solid state, reactions leading to reduction of substituted phenols in various media and radiation treatment of phenols for detoxification purposes. 

The chemical and biological effects of ionizing radiation can be referred to as direct effects when the radiation energy is deposited in molecular targets where chemical transformations are induced. If energy absorption occurs in the external medium (for example water in aqueous solution or biological systems), leading to the formation of radical intermediates which can diffuse to come to react with the molecules, the observed chemical effect is said indirect (Chapter 1). 

Since the use of radioisotopes in nuclear power stations, in anti-cancer radiotherapy or in nuclearweapons, noxious effects of ionizing radiation on human cells are better known. According to the amount and distribution of exposure, ionizing radiation can locally eliminate tumors, but can also damage normal tissues. The biological effects of such radiation result from chemical processes as ionization or excitation of the biological macromolecules, such as DNA, either in a direct way or in an indirect way via water molecule radiolysis (Chapter 12). In both cases, many radical species appear then, which have various consequences on cellular scales such as mutations of DNA, cellular death, or cancer. 

Under irradiation, proteins are affected by direct and indirect effects of ionizing radiations. When these macromolecules are in liquid solution, direct effects can be neglected and the indirect effects are predominant. On the contrary, in solid state, proteins are ionized mainly by direct interaction (11). 

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