Biological systems, ionizing radiation effects

A more reliable means of providing a reference of -OH in a biological system maybe by means of irradiation with ionizing radiation (von Sonntag et al. 2000). The action of ionizing radiation on an aqueous medium gives rise to OH whose yield/dose relationship (G value) is known (Chap. 2). Apart from this, since biological media are concentrated solutions the formation of the indicator product, e.g., a phenol (ArOH), via the direct effect [expressions (69) and (70)] must in principle be taken into account as well. It can be shown that with k4i [probe]/ k42 [cellular components] above 10 4 the direct effect contributes less than 10%... 

The effects of ionizing radiations on chemical and biological systems may be minimized by the addition of certain chemical compounds to the system to be irradiated. These compounds react either directly with the radiation or, more often, with the reactive species produced by the radiations. In so doing, they are themselves transformed into other substances, but their transformation results in the preservation of the integrity of the original chemical or biological system. 

At the chemical level, a solute molecule (DNA, RNA, and protein) in a biological system can be affected by radiation in two different ways. When an ionization track passes either directly through a molecule or close enough so that the created ions can drift to and interact chemically with the molecule before they recombine and neutralize in solution, the phenomenon is called a direct radiation effect. On the other hand, since the largest fraction of almost any biological system consists of water (e.g., 70-80% of a typical cell), the most frequent initial radiation interactions will be with water molecules. When this occurs, ion radicals and free radicals are created. 

A stressor is a substance, circumstance, or energy field that causes impacts, either positive or negative, upon a biological system. Stressors could be as wide ranging as chemical effects, ionizing radiation, or rapid changes in temperature. 

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). 

Effects of ionizing radiation on lipid molecules have been understood by studying model systems which are simpler than the real biological membranes, such as PUFA micelles and liposomes. The formation of lipid oxidative modifications of PUFAs appears as a dynamic process initiated by hydroxyl free radicals generated by water radiolysis, amplified by a propagating-chain mechanism involving alkyl and peroxyl free radicals, and leading not only to hydroperoxides but also to a lot of other lipidic oxidized end-products. Kinetic data, such... 

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