PHYSICO-CHEMICAL METHODS OF PROTECTION AGAINST IONIZING RADIATIONS 



(c) Shielding of a vulnerable group in a molecule {e.g. the prosthetic 

 group of an enzyme) with another substance which can be removed after 

 irradiation. The protection of an enzyme by its substrate^ is believed to 

 function by this principle which will not be considered further here. 



The present paper reports an investigation on the changes produced in 

 a number of synthetic macromolecules under a variety of conditions when 

 protection by different mechanisms was encountered. We hope to be able 

 to establish what type of compounds are most effective in providing pro- 

 tection by the different mechanisms. It may then become possible to deduce 

 by analogy the mechanism of protection in various biological systems by 

 comparing the protective action of a number of substances in vivo with their 

 activity in the synthetic systems. 



COMPETITION FOR FREE RADICALS 



When the action of the ionizing radiations is indirect (i.e. the energy from 

 the radiations absorbed in the solvent produces highly reactive entities 

 which react with the solute) a protecting substance can protect the solute 

 by competitively removing the active entities. Dale^ was the first to find 

 that added substances (notably thiourea) protected enzymes, by a compe- 

 tition mechanism, when these were irradiated in dilute aqueous solution. 

 Dainton'* established experimentally, following the original suggestion by 

 RissE^ in 1929, that the free radicals OH* and possibly H* are formed on 

 irradiating pure water. In the presence of dissolved oxygen the radicals 

 formed are OH' and HOg*. In Dale's experiments the enzymes were 

 probably inactivated by OH radicals^ and the activity of the protective 

 agents in these systems is therefore determined by their reaction with OH 

 radicals. 



The degradation of polymethacrylic acid in dilute aqueous solution by 

 X-rays is due to HOg radicals' and the protective action of more than 100 

 compounds in chis system has been studied^. These compounds protect by 

 competing for HO2 radicals and the protective action is therefore a measure 

 of reactivity with HOg radicals. In another system, the polymerization of 

 methacrylic acid, we have studied protection by competition for OH 

 radicals^. The order of effectiveness of a series of compounds is not the same 

 in the two systems. The activity of substances in protecting mice against 

 the lethal effects of X-rays follows closely the HOg series and not the OH 

 series^' ^. We deduce that competition for HOg radicals plays an important 

 part in the protection of mice. 



protection and DIRECT ACTION 



At first sight it would appear that where the action of the ionizing radia- 

 tions is direct {i.e. the energy is absorbed by the actual material undergoing 

 change) no protection is possible. According to this view, which is widely 

 held, once a macromolecule has absorbed sufficient energy to undergo a 

 chemical change, the ensuing reaction is inevitable and cannot be prevented. 

 Nevertheless an experiment carried out fifteen years ago by Svedberg and 

 Brohult^*' pointed to the possibility that direct action was more complex. 

 These workers found that a very specific dissociation of the giant protein 

 molecule haemocyanin into two equal parts could be induced by irradiation 



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