308 Molecular Action of Ionizing Radiations / 1 6 : 5 



protein changes may be detected with a high sensitivity, but the tests 

 yield no knowledge whatsoever of the intramolecular alterations. 



An exception to this inability to distinguish scission from crosslinking 

 is the molecule, hemocyanin, an iron-containing, oxygen-transport 

 pigment found in snails and other invertebrates. When hemocyanin, 

 molecular weight 7,000,000, is exposed to certain chemical agents such 

 as urea, it undergoes scission, being split reversibly into eight pieces 

 of equal molecular weight. Ionizing radiations also split hemocyanin; 

 however, they split the molecule irreversibly in half. 



One very sensitive criterion for physical changes in a protein is its 

 solubility. Another is its isoelectric point (that is, the jfrH of the medium 

 at which the protein molecule will not migrate in an electrical field) . If 

 the protein is an enzyme, the enzymatic activity is a very sensitive 

 indication of its physical and chemical condition (see Chapters 1 7 and 18). 

 Finally, other proteins react with specific antibodies; in this case, the 

 protein is called an antigen. Its antigenicity may be altered after irradia- 

 tion. Studies on a large number of dried protein films have shown all of 

 the preceding changes. No dried protein films are completely unaltered 

 by ionizing radiation. 



The elimination of small molecules observed with synthetic high 

 polymers can be readily demonstrated for proteins. When either the 

 monomers (amino acids) or their high polymers (proteins) are exposed 

 to ionizing radiation, a number of small molecules are eliminated. These 

 include NH 3 , C0 2 , and CO. In the case of single amino acid mole- 

 cules, the elimination of a smaller molecule represents a scission or a 

 breaking of bonds. An amino acid which does not eliminate NH 3 , 

 C0 2 , or CO during irradiation is cysteine, which contains a sulfhydryl 

 group, — S — H. In the presence of 2 or other oxidizing agents, two 

 cysteines may be oxidized and then may unite to form one cystine. In 

 proteins, there are some indications that such a high fraction of the free 

 — S — H groups are oxidized that energy appears to be transferred 

 preferentially to these sulfhydryl groups from other parts of the molecule. 

 (In pure cysteine, in the absence of any oxidizing agent, H 2 O s and H 2 S 

 are formed under the action of ionizing radiations. Proteins apparently 

 stabilize the free radicals in the cysteine residues so that little or no 

 H 2 S is released.) 



In spite of the indeterminancy of the nature of the changes in most 

 dried proteins, it is possible to investigate energy transfer directly. 

 Such studies are based on the single-hit target theory discussed in 

 Section 4. If one assumes target theory and determines a damage or 

 inactivation versus dosage curve, one can compute a critical volume 

 according to Equation 5. If energy transfer may occur throughout the 

 protein molecule, this critical volume should be the entire molecule. It 



