310 Molecular Action of Ionizing Radiations / 1 6 : 5 



the critical volume determined by antigenic tests corresponds to one- 

 tenth its molecular weight. This implies that severe damage to nine- 

 tenths of the molecule can leave the antigenic activity unaltered. Thus, 

 both catalase and bovine serum albumin may be partially damaged 

 without inactivating the remainder of the molecule. 



Finally, some protein films are able to transfer excitation energy from 

 one molecule to the next. Insulin films show a critical volume corre- 

 sponding to a molecular weight of 23,000, whereas the usual value for 

 the molecular weight is 12,000. This indicates that insulin in these 

 films consists of four units of the type shown in Figure 4, Chapter 15, all 

 linked together sufficiently tightly for energy transfer to occur between 

 them. The digestive enzyme, trypsin, is similar in that it has a 

 molecular weight of about 20,000, but its critical volume corresponds to a 

 molecular weight of 34,000. Thus, energy apparently may be transferred 

 from one molecule to the next. 



If enzyme-substrate complexes (see Chapter 1 7) or enzyme-inhibitor 

 complexes are exposed as dried films to ionizing radiations, the enzymes 

 are inactivated. In these cases, it is found that one ionization anywhere 

 within the complex is sufficient to inactivate the enzyme. Here, energy 

 transfer occurs across the bonds holding the complex together. 



Experiments similar to those with protein films can be done with 

 dried viruses. These have all shown that the critical volume for genetic 

 changes is comparable to the volume indicated by the nucleic acid 

 content. Moreover, they have shown the complexity of the virus, that 

 any gross criterion such as virus multiplication cannot be conveniently 

 described in terms of Equation 5. Rather, to use Equation 5, one has 

 the alternatives of measuring the rate of any genetic change whatsoever, 

 or else comparing the rate of production of the same mutation as 

 indicated by the methods presented in Chapter 14. 



In summary of this section, it should be noted that the inactivations of 

 dried protein and virus films by bombardment with ionizing radiations 

 have indicated several features of the physical properties of proteins and 

 viruses. The similarities between proteins and synthetic high polymers 

 are emphasized by the similarity in the types of radiation damage, 

 whereas the greater complexity of the proteins is emphasized by the 

 greater sensitivity of tests for changes in the proteins. Differences 

 between proteins are demonstrated by the various ranges of energy 

 transfer; in some, the excess energy may be transferred throughout the 

 entire molecule, in others it is restricted to part of the molecule, and in a 

 third group energy transfer could occur between loosely bound pairs of 

 molecules. Finally, the accepted role of the nucleic acids in virus 

 multiplication is in accord with the results obtained through the irradia- 

 tion of virus particles. 



