BIOLOGICAL SYSTEMS 129 



the locus of energy absorption, or it may occur at a more remote region. 

 The latter phenomenon occurs with aromatic compounds. Energy 

 absorbed in the ring may split off a methyl hydrogen from toluene or 

 mesitylene or a methyl group from ethylbenzene (22). Perhaps, in 

 certain conjugated structures, an even more remote split is possible. 

 Thus, "we may conclude that in biological material energy need not 

 necessarily be absorbed in the prosthetic group in order for it to have a 

 devastating effect there. On the other hand, there is no assurance that 

 a purely random hit is necessarily damaging to the prosthetic group. 

 Indeed, we may visualize the possibility that a significant fraction of the 

 volume of a biological material may be damaged (either temporarily or 

 permanently) without effect on the prosthetic group; that is, without 

 lethal effect as we might measure it. We might expect, consequently, 

 that the probability of a chemically effective hit increases with the 

 number of nearly simultaneous hits made on a particle of biological 

 material. Seemingly, our best evidence (28, pp. Ill ef seq.) is that in 

 many cases a single hit is all that is necessary. A multiplicity of hits, 

 as by an alpha particle, seems to be no more effective than a single hit, 

 as by an electron. However, this conclusion is based on calculation and 

 surmise — certainly not on direct visual observation — and there is no 

 requirement that the hit be within the biological particle. A verj^ care- 

 ful analysis of the implications of this so-called tai^get theory in the light 

 of our knowledge of the elementary processes of radiation chemistry is 

 necessary for an understanding of the data and of the attendant processes. 



SOME REMARKS ON THE TARGET THEORY 



It is by no means the function of this paper to attempt a critical e^•alu- 

 ation of the target theory, details of which have been presented so ably 

 by Lea (28). Rather, I would prefer to interpret certain aspects in the 

 light of our general knowledge of radiation chemistry. In his presenta- 

 tion Lea recognized quite clearly that the not-necessarily spherical 

 target had dimensions which were not necessarily identical with the 

 particle of biological material (28, pp. 93 et seq.). Since ionization in the 

 water immediately surrounding the particle could produce free H atoms 

 and OH radicals capable of reaction with the biological material, the 

 target size according to L. H. Gray is effectively increased by an amount 

 related to the diffusion distance of those active particles (28, pp. GG, 67). 

 Lea stated that this distance could be 150 A for H atoms but only 20 or 

 30 A for OH radicals. He neglected mention of the diffusion distance of 

 the very much more persistent HO2 radicals. However, perhaps the 

 best way to see the target is to look at it as a whole, that is, the biological 

 particle and its environment. 



