292 RADIATION BIOLOGY 



interpretation of the ionic yield as a cluster of 21 acetylene molecules is 

 untenable (Rosenblum, 1948). The radiochemical polymerization of 

 acetylene is regarded as proceeding by means of successive bimolecular 

 reactions between a normal acetylene molecule and an excited molecule 

 or polymer: 



C2H*.^ (C2H2)r-£!H4 (C^H^)* 



where CoH.f is an excited acetylene molecule or radical and (C2H2)* and 

 (C2H2)* stand for excited polymers. 



Another approach to the interpretation of radiochemical reactions in 

 biological systems is being made by the study of reactions occurring in 

 gels (Day and Stein, 1950). When dyestuffs contained in gels are 

 irradiated, the dye is reduced. Oxygen if present competes with the 

 dye for the reducing agent: dye + e -^ (dye)"; O2 + e — > O^. The elec- 

 tron is that produced on irradiation of water: H2O -^ H2O+ + e. The 

 relative extent of these reactions will depend on the various electron 

 affinities, and it is possible that very low concentrations of a suitable 

 acceptor may capture practically all the electrons. 



EFFECT OF IONIZING RADIATIONS ON PROTEINS 



The complexity of the protein molecule, its large size, the number of 

 side groups with different degrees of reactivity, and its shape (which 

 permits changes without alteration of its molecular weight) are the 

 reasons for the different mechanisms of action of ionizing radiations. 



When dilute solutions of protein are irradiated with small doses of ion- 

 izing radiation the effect may be entirely due to those produced by the 

 radiation products of water, i.e., oxidation of sulfhydryl or hydroxyl 

 groups and perhaps of amino groups and polymerization. When the 

 protein concentration is increased or the radiation dose is high, to the 

 indirect effect there may be added the direct collision of the ionizing 

 particles mth the protein molecule itself, causing rupture of hydrogen 

 bonds, rupture of side chains, denaturation and splitting of the molecule, 

 and precipitation. 



Oxidation of the sulfhydryl groups is one of the first reactions. This is 

 a reversible reaction which proceeds with high efficiency and can be best 

 studied by measuring the sulfhydryl groups on irradiation of proteins. 

 If these sulfhydryl groups are important for the functional activity of 

 the protein, inactivation will ensue. The hydroxyl groups of tyrosine, 

 hydroxyproline, and /3-hydroxyglutamic acid and the double bonds of 

 histidine are also easily oxidized. Oxidation of the tyrosine groups can 

 be followed easily by spectrophotometric measurements in the ultra- 

 violet region. Increase in the light absorption on irradiation was 

 observed as long ago as 1927 (Spiegel- Adolf and Krumpel, 1927). These 



