PHYSICAL PRINCIPLES OF CHEMICAL REACTIONS 245 



pended in the water, the peroxide will react with it and may impede or 

 destroy its function as a result of various oxidation reactions. It is now 

 established as a rather general result in a variety of experimental circum- 

 stances that the damaging influence in such cases is strongly enhanced by 

 the presence of oxygen, whereas (preirradiative) addition of easily 

 oxidizable material which removes peroxide has a protective influence. 

 Not only hydrogen peroxide formation in the water, but also products of 

 reactions involving certain other solutes, can cause indirect damage. 

 Thus, to cite one example, according to Barron (private communication) 

 certain biological material suspended in sea water is more sensitive to 

 high-energy radiation than if suspended in pure water. Reaction of OH 

 radicals with hydrated halide ions, by which halogen atoms (and, sub- 

 sequently, molecules) are formed (charge transfer in collisions in solution; 

 cf. Sect. 4-4) might be responsible for this extra damage. 



The use of the word "poison" for an indirect influence would seem to 

 be a proper one in many cases, for if the biological material is in dilute 

 solution the only way in which primary products of the irradiation formed 

 in the environment can have an influence is by actual diffusion of an 

 energy-rich entity (such as H2O2) formed by primary or subsequent 

 interaction of the radiation with the environment, and the only entities 

 that can have a significantly great diffusion distance are those that are 

 appreciably stable. The "poison" itself need not, however, be a chem- 

 ically stable substance, for some free radicals (such as HO2 and I) formed 

 secondarily by reaction of primary products with solutes (O2, I~) can live 

 long enough to have an appreciable radius of influence. ^^ Other free 

 radicals, as for example the primary ones H and OH, may be very reactive 

 but are, therefore, short lived; they can effect a change only if produced 

 in the immediate neighborhood of the biological entities. Thus the 

 indirect type of influence verges on the direct, and in such circumstances 

 no attempt should be made to force the interpretation into unique con- 

 formity with one or the other hypothesis. 



5-2. DIRECT CHEMICAL CHANGES PRODUCED BY 



HIGH-ENERGY RADIATIONS IN MOLECULES 



OF BIOLOGICAL IMPORTANCE 



If a swiftly moving charged particle passes through a complex of bio- 

 logical material, or even if it passes in the immediate vicinity, the bio- 

 logical function of the complex can be altered or even destroyed. Radio- 

 biologists speak in this circumstance of the influence of a direct hit on 

 a biological target. 



21 There may even be chain reactions. A suggestive possibiHty is 



HO2 + RH -^ HROo + H 

 H + O.^ HO2 

 Such a chain would cause both heating and a partial "burning " of the organic material. 



