Biological Effects of Penetrating Radiations 91 



Radiochemistry 



In studying the effects of radiation on biological material use- 

 ful information may be obtained from experiments on nonliving 

 matter. A recent survey by Allsopp - of the chemical action of 

 radiations has shown how developments in the field of radio- 

 chemistry can be related to the study of the biological effects of 

 radiation. Until quite recently, enormous doses of radiation 

 were required to produce measurable chemical changes in vitro, 

 and it w^as suggested that chemical processes could not be in- 

 volved in therapeutic radiation at any rate, since recognizable 

 changes could be obtained only with doses far above the max- 

 imum human tolerance dose.^^ 



Recent work by Dale,^^' -^' ^^ however, has shown the fallacy 

 of the conclusion. Dale arranged his experimental procedure so 

 that the chemical changes produced by irradiating purified 

 enzymes in aqueous solution were magnified many times by the 

 accompanying changes in biological activity. Dale's results show 

 quite clearly that a constant amount of solute is inactiviated for 

 a given amount of radiation energy absorbed in the whole solu- 

 tion, irrespective of the concentration of the solution. The 

 simplest explanation of these results is that the initial process 

 consists in "activation" of solvent molecules by absorption of 

 radiation, followed by the transfer of energy to the solute by 



inelastic collision, without the term "activation" being precisely 

 defined.40. 112 



It may be recalled here, however, that in the initiation of 

 radiochemical reactions in gaseous systems, excitation of mole- 

 cules is apparently more important'than ionization, since radio- 

 chemical reactions in the gas phase in general follow the same 

 course as the corresponding photochemical reactions. ^^' ^^' ^-- 

 There is no reason to suppose that radiochemical reactions in 

 aqueous solutions are not similarly initiated by energy-carrying 

 solvent molecules.^ The experimental evidence is consistent with 

 the hypothesis that the energy carrier is a free hydroxyl radical. ^^^ 



Since the number of solute molecules decomposed by a given 



