16:5/ Molecular Action of Ionizing Radiations 

 5. Inactivation of Dried Protein Films 



307 



When dried protein films are subjected to ionizing radiation, the mole- 

 cules are irreversibly altered. Because no solvent is present, the changes 

 observed are of necessity direct ones in the protein itself. In most of the 

 experiments described in this section, no attempt was made to exclude 

 oxygen or determine its role, if any, in the final molecular alterations. 

 Protein films are usually tested for molecular changes after redissolving 

 them in suitable media (usually buffered water) . Tests of the physical 



Lines for n/n Q = e SD 

 Equation (I) 



S=2.5 



Ionization /Unit Pathlength 

 (b) 



Figure 5. Curves illustrating the inactivation theory, (a) 

 Curves show the predicted relationship for the number of 

 particles remaining at different ionizations per unit path length. 

 These were used to plot S below for the curve S — 10. The 

 ratio n/n is the fraction of the molecules remaining unaltered 

 after exposure to dose D. (b) Predicted curves for finding the 

 critical volume V and limiting the cross section S . To deter- 

 mine these curves experimentally necessitates a series of experi- 

 ments such as those illustrated in (a) . The straight lines near the 

 origin are predicted by Equation (5), whereas those parallel 

 to the axis at the right edge of the graph are predicted by 

 Equation (6). 



and chemical properties of proteins are far more sensitive to any small 

 change whatsoever than are those used on synthetic polymers. However, 

 in most cases, even though a change can be detected, it is not possible to 

 determine whether crosslinking or scission has occurred within the 

 protein molecule. The polypeptide chains making up the protein 

 appear to be so crosslinked that either scission or additional crosslinking 

 can occur without altering the molecular weight. In other words, 



