246 RADIATION BIOLOGY 



The possible influences of "hits" are understandable, since there can be 

 ascribed to biological material the same general sorts of chemical changes 

 as those induced by excitation and ionization processes in ordinary 

 chemical substances. There is one essential difference, however, namely, 

 that the molecules and molecular groups of biological significance are 

 much more complex than ordinary chemical compounds, and, therefore, 

 not only are many more types of chemical change possible in the former, 

 but there is far less specific knowledge from spectroscopy, photochemistry, 

 and radiation chemistry available to aid in their understanding. 



It is not the function of this chapter to analyze the evidence for the 

 target mechanism in any particular instance. The question of direct vs. 

 indirect influence is for most cases one still very much in a state of active 

 exploration and lively debate. At best, only few examples of the direct 

 hit mechanism can be considered to have been conclusively established, 

 and proponents of each of the alternatives have often overextended the 

 models in applying them to actual experiments. It cannot be doubted 

 that, as applied, the models are usually vastly oversimplified idealizations, 

 and that in testing them in experimental situations, quantitative sig- 

 nificance has all too commonly been attributed to arguments that are at 

 most of qualitative validity. 



Nevertheless, if the understanding of the target hypothesis is to 

 advance, one must press for a realistic analysis of the actual phenomenon 

 of the "hit" on a valid physical basis. The mere statement that "a hit 

 deactivates or modifies a system" poses a problem but does not close a 

 question. In the following paragraphs a general discussion with several 

 examples is given as a means of illustrating the initial steps which such 

 an analysis might follow. 



If the biological material be considered to be composed of single large 

 molecules or molecular aggregates, then the question devolves on the 

 effect of excitation or ionization on such an entity. Isolated single events 

 are considered first. Excitation can be followed by luminescence, by 

 dissociation or predissociation, or by internal conversion." The first 

 possibility can be disregarded because it is so rare in the type of poly- 

 atomic molecule most commonly found in biological systems. (When 

 present it can usually be readily recognized.) Either type of dissociation 

 will split off an atom or radical, and this may have a permanent effect if 

 the reverse reaction has a small probability. Such primary recombina- 

 tion is favored by the cage effect in all biological systems, but opposed by 

 the circumstance that the remainder of the molecule, being still a very 

 large molecule, may tend to reorganize slightly and thereby cause the 

 activation energy for the recombination to be great enough to hinder the 



22 In considering target mechanisms in biological media, only internal molecular 

 transformations, which are monomolecular processes, need be treated. However, 

 energy migration may occur (of. Sect. 3-4d). 



