PHYSICO-CHEMICAL METHODS OF PROTECTION AGAINST IONIZING RADIATIONS 



We have no evidence that hydrogen transfer plays a part in biological 

 protection. Prevot-Bernas^i observed that cysteine and cysteamine act as 

 chain terminators in polymerization processes by hydrogen transfer and 

 postulated that the protective action of these substances in mice was due to 

 their ability to act as transfer agents and not as competitors for free radicals. 

 This hypothesis can be rejected for the following reasons, (a) Many pro- 

 tective agents which are active in vivo with mice and in vitro with polymeth- 

 acrylic acid do not act as transfer agents in polymerization reactions under 

 conditions where the — SH compounds do.^ {b) Amines are as active as 

 protectors in the cationic as in the un-ionized form whereas they can only 

 transfer a hydrogen atom when un-ionized^. Crosslinking of polyvinyl 

 alcohol is prevented by un-ionized methylamine but not by ionized methyl- 

 amine. This further supports the view that in this system protection occurs 

 by transfer, (c) All — SH compounds act as transfer agents while only very 

 few are capable of protecting mice^. 



protection and TARGET THEORY 



In the earlier formulation of the target theory (see for example Lea^^) a 

 chemical change was postulated to occur whenever an ionization had 

 occurred ; the energy for this event plus its associated excitation was assumed 

 to be the same as that required to ionize an atom in air {i.e. about 32 eV). 

 Current radiochemical research has shown that this second postulate need 

 not apply and that when pure materials are irradiated a chemical change 

 often occurs only when very much more than 32 eV of energy has been 

 absorbed. We have recently shown {see page 54) that for some reaction 

 such as the breaking of main-chain bonds in polyisobutylene considerably 

 less than 32 eV may be sufficient to produce a discrete chemical change. 

 Thus, if radiation of low specific ionization were used to determine a mole- 

 cular weight from the apparent ' target size ' in, for example, polystyrene 

 (using crosslinking as a criterion), a very low value would be obtained, 

 while with polyisobutylene the molecular weight derived would be too high. 

 These experiments may make it necessary to re-evaluate the data obtained 

 for the sizes of the sensitive volumes of viruses and enzymes from ' target 

 area' calculations, since the chemical change may not be confined entirely 

 to the track of the ionizing particle. In particular the occurrence of energy 

 transfer throws doubt on the validity of the calculation for the number of 

 sensitive sites in an irradiated organism from the variation in apparent ' target 

 size' (or relative biological effectiveness, RBE) with the specific ionization 

 of the radiation used. This problem has recently been dealt with in great 

 detail by Zirkle and Tobias " who interpret the RBE of different radia- 

 tions in terms of the diffusion of free radicals. Transfer of energy initially 

 deposited at certain points may provide an alternative explanation for the 

 observed facts. 



THE mechanism OF ' DIRECT- A CTION ' 



Largely as the result of Dale's work direct and indirect action have been 

 clearly differentiated. The least ambiguous test for distinguishing between 

 the two processes is to examine how the percentage inactivation, of for 



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