6 : 5/ Molecular Action of Ionizing Radiations 



309 



is likewise conceivable that energy transfer could occur in only part of 

 the molecule or, at the other extreme, energy transfer may take place 

 between adjacent molecules. These conditions could lead to critical 

 volumes which would be small or large, respectively, as compared to one 

 protein molecule. 



All three possibilities — critical volume equal to, smaller than, and 

 larger than one molecule — have been observed by using dried films of 

 different proteins. The case of the critical volume equal to the molec- 

 ular volume has been observed for DNA-ase, invertase, and many other 



Electron data 

 Deuteron data 



Sq extrapolated from 

 electron data for a 

 spherical molecule 



10 20 30 40 50 60 70 80 

 Energy Loss (ev/mji,) 



Figure 6. The critical volume and limiting cross section for 

 DNA-ase. Points are shown for deuteron data only. After 

 R. Setlow, ''Radiation of Proteins and Enzymes," Annals of 

 the New York Academy of Sciences 59: 471 (1955). 



proteins, as well as for one smaller molecule, penicillin, molecular weight 

 600. A plot of typical experimental data is shown in Figure 6. Al- 

 though energy transfer throughout the molecule is a sufficient condition 

 for the equality of the molecular and critical volumes, it is not a necessary 

 condition. It is possible that any of a variety of small changes in 

 various parts of the molecule would all lead to the same conclusion of 

 protein damage. Thus, finding a critical volume which equals a 

 molecular volume suggests that energy transfer may take place through- 

 out the entire molecule but does not prove it. 



In contrast, a critical volume smaller than the molecular volume 

 proves that energy transfer cannot occur throughout the entire molecule. 

 This is the case for catalase which has a critical volume, as determined by 

 tests using enzymatic activity, corresponding to one-half its molecular 

 weight. (If urea is added to the test medium, it is found that the critical 

 volume corresponds to the entire molecular weight.) Bovine serum 

 albumin is a more extreme example. When a monolayer is irradiated, 



