EFFECT OF X EAYS ON BIOLOGICAL SYSTEMS 291 



production of hydroxyl radicals in such spatial proximity that multiple 

 attacks on the same molecule become more Hkely. 



Irradiation of nitrobenzene with X rays produces o-, m-, and p-nitro- 

 phenols, the ratio of these isomers being essentially the same as those 

 obtained on hydroxylation with OH and O2H radicals with Teuton's 

 reagent (Loebl et al., 1950). 



As might be predicted from the great reactivity of the free radicals pro- 

 duced on irradiation of water, cholesterol is attacked by X rays with 

 oxidation of double bonds in the 5 : 6 position leading to the formation of 

 the corresponding (trans) triols (Keller and Weiss, 1950). When choHc 

 acid (0.5 per cent aqueous solutions of sodium cholate) is irradiated with 

 1.8 X 10^ r at 35°C, two oily fractions can be isolated, one of which 

 gives, after acetylation, methyl-3: 12-diacetoxy-7-keto cholanate. This 

 substance is also formed in the first stages of the biological oxidation of 

 cholic acid (Keller and Weiss, 1951). 



An interesting reaction is the hydrolysis of carbon tetrachloride on 

 irradiation of aqueous solutions. The production of hydrogen chloride is 

 directly proportional to the X-ray dose. When irradiation is performed 

 in the presence of alcohol, there is inhibition and the rate of hydrolysis 

 becomes exponential (Minder et al., 1948). 



Polymerization reactions can be brought about by radical reagents, 

 probably by way of the formation of a radical of the type (R2C — CR2)^ 

 and its condensation according to the equation 



(R2C— CR2)- + R2C=CR2 -^ (R2C— CR2— CR2)- 



forming a new radical that may condense further. The polymerization 

 stops by disappearance of the odd electron through combination of 

 radicals, by exhaustion of material, or simply because the entropy of 

 activation decreases with the increase of polymerization. These poly- 

 merization reactions are of great importance in radiobiology because 

 many effects produced by ionizing radiations are polymerizations (pro- 

 tein precipitations, chromosome "aberrations"). Polymerization of 

 methylmethacrylate in aciueous solutions by hydroxyl radicals produced 

 chemically depends upon the hydroxyl radical concentration (Baxendale 

 et al, 1946). Methylacrylate, styrene, and vinyl acetate are readily 

 polymerized on irradiation by 7 rays, /3 particles, or neutrons (Hopwood 

 and Phillips, 1939; Dainton, 1948). The 7-ray-induced polymerization 

 of acrylonitrile in dilute solutions is a reaction of the second order with 

 respect to the monomer concentration, and the rate appears to be inde- 

 pendent of the source strength. In the polymerization of acetylene 

 under the influence of a rays there is considerable formation of benzene. 

 The over-all ionic yield, M/N, is 21. On the assumption that one-fifth 

 of the reacting acetylene produces benzene, it may be concluded that 



