BIOLOGICAL SYSTEMS 127 



Thus, presence of dissolved oxygen in a biological system sensitive to 

 oxidizing agents makes that system more sensitive to the effects of 

 radiation. An antidote to oxygen in biological systems involves the 

 incorporation in such systems of strong (non-toxic) reducing agents not 

 quite capable, however, of direct reaction with oxygen. This latter 

 requirement is not essentially a thermodynamic one (for example, the 

 redox potential might even permit a direct reaction with oxygen) but 

 in reality one of kinetics (that is, the reaction with oxygen simply should 

 not proceed under the particular conditions). Choice of suitable com- 

 pounds depends primarily on an experimental search. 



Oxygen, however, does not always play a special role. Imagine an 

 alpha-irradiation process in which a hit is scored directly within the 

 biological particle. Since the alpha is relatively slow moving, there will 

 be a number of accompanying hits in, and close to, the particle. As a 

 result, some free radicals are necessarily produced so close to the bio- 

 logical particle that their only fate can be to react with it. Consequently, 

 it would not be inconsistent with these simple concepts to discover that 

 presence or absence of oxygen is without discernible effect on the results 

 obtained in alpha irradiation of biological systems. 



CHAIN REACTIONS 



WTien a radical such as OH or HO2 reacts with an organic compound, 

 it either breaks a single bond or opens a double bond. In either event a 

 new free radical is produced, the fate of which depends on its specific 

 chemical properties. It is important to appreciate that under suitable 

 conditions (for example, when the initial process is distinctly exothermal) 

 this free radical may itself enter into chain reactions. One such type of 

 chain is common to pyrolytic reactions: 



R— + AH -^ RH + A— 



A > R— -f S 



The radical R reacts with species AH in a reaction which requires activa- 

 tion energy. Decomposition of A into R plus a stable product S may 

 also require activation energy. Such chains would tend to be very short 

 near room temperature and would probably not be very important in 

 biological processes. Another type of chain is characteristic of the 

 growth of polymer molecules: 



R h A -^ RA— 



RA 1- A -> RA2— 



RAn 1- A — > RA„+i — 



