312 Molecular Action of Ionizing Radiations / 1 6 : 6 



H 2 2 and free radicals than are the proteins and nucleic acids. Pro- 

 tective agents of this type can also reduce damage due to direct effects 

 of the ionizing radiations because the extra energy may be transferred 

 to the protective agent from the protein or nucleic acid. Among the 

 most effective compounds of this nature are a group containing both 

 — S — H or — S — S — groups and amino groups — NH 2 , such as cysteamine 

 and cystamine. These compounds protect not only proteins and 

 nucleic acids, but also synthetic condensation-type polymers such as 

 polymethacrylic acid. Other protective agents with very different 

 structures have also been used, such as /S-aminoethyl-isothiuronium- 

 bromide-hydrobromide. 



In terms of critical volumes, the net effect of the protective agent 

 will be to reduce the critical volume far below the molecular volume. 

 Various theories have been developed to explain the action of the pro- 

 tective agents on proteins as due to stabilizing terminal — S — H and 

 — S — S — groups. However, because the same agents also protect syn- 

 thetic polymers which do not contain sulfur, it appears unjustified to 

 focus too much attention on one particular type of bond. 



Studies have been conducted on the relative sensitivities of many 

 amino acids, proteins, and nucleic acids to ionizing radiations. These 

 have led to quite lengthy tables, but no one has succeeded in relating 

 this information to protein structure. The relative sensitivities of 

 different proteins can be indicated by a G value; this is the number of 

 molecules altered per 100 ev (of a fixed type of irradiation). Some 

 typical values are shown in the accompanying table which indicates the 

 widespread range of G values. 



The values in the table show that as the complexity of the molecule 

 increases, there is some tendency for the molecule to become less sensitive 

 to ionizing radiations. Thus, adenine is more sensitive by itself than 

 when combined with ribose to form the nucleoside, adenosine. It in 

 turn is more sensitive than the nucleotide, adenosine monophosphate 

 (adenylic acid). Complete nucleic acids are another order of magnitude 

 less sensitive. However, the variation of sensitivity with size is not 

 always observed. Some proteins are almost as sensitive as a typical 

 small molecule, whereas others are far less sensitive, so that no general 

 rule can be maintained. 



TABLE I 

 G Values for Biologically Active Molecules* 



