PHYSICO-CHEMICAL METHODS OF PROTECTION AGAINST IONIZING RADIATIONS 



protection is now known to be possible against direct action. In addition we 

 have observed that the absorption spectrum {see Figure 2) of polyisobutylene 

 is modified much more extensively after irradiation under nitrogen than 

 under air''''. This is a clear example of an oxygen effect when the action 

 is direct since exposure of the polymer to oxygen after irradiation does 

 not modify the absorption spectrum. We can conclude that irradiation 

 produces different chemical products in the presence and absence of 

 oxygen even where the process is ' direct '. 



The degradation of both polyisobutylene^'' and of polymethylmethacryl- 

 ate^^ depends on temperature. The energy required to break a main-chain 

 bond in polyisobutylene is nearly doubled on decreasing the temperature 

 from 70° C to — 60° C. A possible interpretation is that transfer of energy 

 into the required bond is more efficient at higher than at lower tempera- 

 ture {cf. fluorescence) . A similar observation has been made with biological 

 materials. Dry enzymes^* and dry bacteriophage^^ were reported to be 

 inactivated more readily {i.e. their target size appears to be larger) at higher 

 temperatiu-e. This has been interpreted by Pollard-* in terms of an 

 inherent change in sensitivity of the enzyme with temperature and by 

 Bachofer et al^^ as due to the diffusion of an active intermediary within 

 the dry phage. Since a temperature dependence has now been found with 

 a completely synthetic material where neither of these explanations can 

 apply, it may be necessary to assume an energy transfer process. Although 

 these experiments in no way disprove that the action of ionizing radiations 

 on living systems is indirect they show that an important contribution by 

 direct action cannot be excluded. Since in the cell and particularly in the 

 cell nucleus proteins and nucleic acids are present in relatively high concen- 

 trations, it has always been difficult to understand why the action of ionizing 

 radiation should be predominantly indirect. 



SUMMARY 



The possible ways in which the primary chemical effects of ionizing radia- 

 tions can be reduced are reviewed. 



Examples are given of protection against indirect action by competition 

 for free radicals and of repair of a damaged macromolecule by transfer 

 agents present during the irradiation. Irradiation of solid polymers has 

 shown that protection is possible against direct action and an undefined 

 energy transfer process is suggested as the mechanism. Such an eflfect will 

 require modification of the 'target theory' in its more general form. 



In addition to protection by added chemicals, direct action can also be 

 influenced by the presence of oxygen and by changes in temperature. 

 This shows that 'direct' action need not be an unalterable event and that 

 it may play a more important part in the irradiation of living systems than 

 is generally assumed. 



This work has been supported by grants to the Chester Beatty Research Institute from the 

 British Empire Cancer Campaign, the Jane Coffin Childs Memorial Fund for Medical Research, 

 the Anna Fuller Fund, and the National Cancer Institute, National Institutes of Health, U.S. 

 Public Health Service. 



58 



