BASIC RADIATION BIOCHEMISTRY 257 



these processes, their respective HkeHhoods, and their equilibrium states 

 are treated by Barron in Chap. 5. 



This short discussion of the radical theory forms the basis for an under- 

 standing of the reactions occurring in aqueous solutions. The modern 

 theory of radicals is a relatively recent development. In order to put it 

 into its proper perspective, a short review of this development seems 

 expedient. 



The most conspicuous feature of "ionizing radiations" — as indicated 

 by their name — is the fact that these radiations ionize matter through 

 which they pass. Since the outer electron shell of atoms determines the 

 chemical properties of these atoms and of the molecules they constitute, 

 the disturbance of electrons can result in a subsequent rearrangement in 

 the molecule concerned, i.e., in chemical change. Up to twenty years 

 ago the study of ionization was carried out mainly on gaseous systems 

 because it is easier and more precise to measure the radiation effects in 

 the gas phase than in solution. Such experiments were therefore in the 

 majority. In the gaseous state, however, there is no solvent, and the 

 changes observed are caused by direct transfer of radiation energy to the 

 gas molecules themselves. Gradually research turned to solutions, but 

 investigators conditioned by the observations on gaseous systems 

 neglected the fact that a solution consists of a solvent as well as a solute 

 and thereby also neglected the possibility' that the primary events occur- 

 ring in the solvent could have consequences for the solute. Thus in 

 experiments on solutions attention was focused on the direct ionization 

 of the solute molecules only, these being conceived of as the target of the 

 ionizing particles. 



Mention of the formation of atomic hydrogen and OH radicals was 

 probably first made in a paper by Risse (1929) on the effect of radiation 

 on water. After that the radical theory remained in a more or less latent 

 state, but the extensive work of Fricke and his school (1929 to 1938) on 

 aqueous solutions of inorganic and simple organic compounds and the 

 work of Dale (1940, 1942, 1943) on enzymes and other biologically active 

 substances clearly established that the action of radiation on the solvent — - 

 water — had an effect on the solute. Fricke called this particular agent 

 of the radiation effect "activated water." The further development of 

 the idea that "activated water" could be identical with H atoms and 

 OH radicals, formed by radiation, is due to Weiss (1944). Both Weiss 

 and Dainton have provided experimental proof, in certain cases, that 

 these entities take part in radiation chemical reactions [e.g., hydroxyla- 

 tion of benzene (Stein and Weiss, 1949); polymerization of acrylonitrile 

 (Dainton, 1947)]. 



These developments have established the "indirect action" theory of 

 ionizing radiations based on the formation of an intermediate agent 

 (activated water, OH radicals, and H atoms) which in turn acts on the 



