IONIZING RADIATION AND VIRUSES 89 



is 20, as 10^ per sec if it is 100,000, and as 2 X 10^ per sec if it is 

 10'^. Thus for heavily ionizing particles, recombination takes 

 place faster than collisions with large molecules, but for elec- 

 tron-produced radicals, collisions even with virus particles 

 may occur before recombination takes place. Thus the radiation 

 produces active agents of short half-life which can be transported 

 through the water and which can be active chemically. This 

 chemical action was discovered by Fricke (1927, 1934) and 

 neglected by radiation biologists for several years. The dis- 

 covery of radiation action in dilute solutions of enzymes by 

 Dale (1940), and of the great sensitivity of sulfhydryl groups 

 by Barron and Dickman (1949), has stirred up great interest in 

 this process. Its role in virus inactivation is important but 

 strongly dependent on the conditions of irradiation. 



Three major factors about this class of indirect radiation 

 action must be remembered. First, there is no reason to believe 

 the one radical will inactivate a virus, particularly a virus such 

 as T-1 which can survive more than 1,000 ev of primary ioniza- 

 tion action. So the number of radicals necessary to inactivate 

 must be known. Second, the increased collision rate with 

 smaller molecules renders these very effective competitors for 

 radicals, particularly so if they are small molecules that can 

 react chemically with free radicals. Cysteine and glutathione 

 are such molecules. Hence, if any excess concentration of such 

 molecules, or even of gelatin, is present, competition will remove 

 radicals by combination with these substances and so protect 

 the virus. This protective action was discovered by Friedewald 

 and Anderson (1940), Luria and Exner (1941), and Dale (1942). 

 The third factor is that radical action takes place on the surface. 



All these factors can be made use of in studying viruses. 

 Estimates of the "radical yield" are not easy because it is 

 first necessary to know the number of radicals per ionization. 

 Taking this to be unity, which is reasonable for electrons, 

 figures given by Lea (1947) lead to a figure of 4,000 radicals per 

 inactivation of tobacco mosaic virus. It is quite likely, as 

 McLaren (1951) has pointed out, that radical yield is dependent 

 on virus surface area. 



