74 BACTERIOPHAGES 



3. Ionizing Radiations 



Most investigators of the effects of ionizing radiations, i.e., 

 radiations belonging to the shortest wavelength sector of the 

 electromagnetic spectrum, have been interested in the interpre- 

 tation of inactivation data in terms of the "target theory." It 

 was hoped that development of the target theory would yield in- 

 formation about the size and shape of the radiation-sensitive 

 structures in viruses (Latarjet, 1 946 ; Lea, 1 946) . Some of the 

 calculated target sizes have already been presented in Chapter 

 IV, and their relation to phage particle sizes determined by 

 other means has been discussed. Recently, an increased em- 

 phasis has also been placed on the study of the physiological 

 effects of ionizing radiations on phage particles, stimulated, no 

 doubt, by the amazing variety of effects produced by ultraviolet 

 (nonionizing) radiation. 



Extensive discussions of the chemical effects of ionizing radia- 

 tions and possible mechanisms for their biological action can be 

 found in Lea (1946) and in Hollaender (1954). We may state 

 briefly here that the effects of ionizing radiations may be classed 

 under two categories: direct and indirect. Direct effects are 

 produced when an ionization occurs directly in the substance 

 under investigation, whereas indirect effects are caused by chemi- 

 cal agents produced by the action of ionizing radiations on the 

 solvent molecules. Irradiation of pure water produces highly 

 reactive but unstable free radicals, such as OH, H, and OoH, as 

 well as more stable peroxides. The presence of dissolved oxygen 

 increases the toxic and mutagenic effects of ionizing radiations, 

 while the presence of sulfhydryl compounds and certain simple 

 organic substances reduces these effects. 



The irradiation of dry substances will result in direct effects 

 only, since there is no solvent. Irradiation of dilute solutions 

 will result in both direct and indirect effects, while irradiation of 

 concentrated solutions will result in predominance of direct 

 effects and a relative decrease in indirect effects. The addition 

 of certain kinds of solutes will protect against the indirect effects 

 because the added solute molecules will react preferentially with 



