302 Z. M. BACQ AND P. ALEXANDER 



living beings (Bacq and Alexander. 1961 ; Bacq, 1957, 1959; Patt, 1953, 

 1954; Pihl and Eldjarn, 1958; van Beklaim and Zaalberg, 1960). 



We shall limit onr discnssion not only to a few substances : cysteamine 

 (H S — CH2 — CH2 — NH2 and some related compounds), but also to few 

 systems (large molecules in vitro and mammals). 



PROTECTION AT THE CHEMICAL LEVEL 



Polymers and proteins are quite suitable for determining the mechan- 

 isms of chemical protection because the conditions of the experiment 

 (oxygen pressure, temperature, water content, concentration of the 

 protector, quality of the radiation, etc.) can be drastically altered and 

 accurately known, which is far from being the case with mammals. 



Radiation damage to a given molecule can be prevented in one of 

 three ways : 



1 . By diverting the absorbed energy. If the damage is being caused by 

 indirect action then the protector intercepts the fi'ee radicals 

 formed in the water and prevents them from reacting with the 

 receptor. If the damage is due to direct action then the energy 

 deposited in the macromolecule is transferred to the protector 

 before it has time to cause chemical changes. 



2. By repairing the dama{/ed molecule (Alexander and C'harlesby, 

 1955a). After becoming ionized, or reacting with the OH radicals 

 formed by the ionization in the surrounding water, a molecule 

 is not necessarily irreversibly damaged. A radical can be formed, 

 with a lifetime in swollen systems of some 10"^ sec, which can 

 react with a protector in such a way as to be restored. 



For example, 



RH becomes ionized and 



target molecule > R° (direct action) 



then dissociates 



RH+OH° R°+H20 



radical from > (indirect action) 



water 



In the absence of a protector R° reacts further and becomes irre- 

 versibly damaged, while by combining with the protector (PH) 

 it is restored or rei)aired. 



R° -t-PH ^ RH -f po 



Oxygen, on the other hand, tends to enhance radiation damage 

 because it combines with R° to give a radical R02° which is no 



