PHYSICAL PRINCIPLES OF CHEMICAL REACTIONS . 247 



primary back reaction. No general conclusions as to these opposing 

 tendencies can be made, and instances of either extreme may be expected. 

 How great the biological influence of an unreversed dissociation will be 

 cannot of course be answered generally, but it seems likely that some 

 varieties of radiobiological effect should arise from this process. How- 

 ever, internal conversion may be expected to be the most common conse- 

 quence of electronic excitation in a molecule of biological importance. It 

 will almost always bring the molecule to a state of high oscillational energy 

 and thereby, by communication of this oscillation energy to the environ- 

 ment as heat, to the normal state. Thus excitation will commonly have 

 no permanent effect. If the "hot molecule" should be structurally sen- 

 sitive to its high "internal temperature" a permanent change might, how- 

 ever, result. (This suggests a correlation between thermal and radiation 

 sensitivity.) Of particular interest is the special case of permanent 

 molecular reorganization following the internal conversion, to be dis- 

 cussed in succeeding paragraphs. 



A single ionization should often be reversed by subsequent recapture 

 of an electron. This is not to imply that there will be no permanent 

 effect, however, for the electron will be captured so as to produce a highly 

 excited electronic state of the molecule. Thus the energy liberated 

 during the recombination will be small, whereas the rest of the energy is 

 dissipated in a way usual for excitation energy, namely, through dissocia- 

 tion of the molecule or as oscillational energy. There is also the additional 

 possibility that the primary ion dissociates into two or more fragments 

 (Sect. -i-Sb), and if this occurs the probability of reversal will usually be 

 small; the process should, therefore, be a common one in mechanisms of 

 radiobiology. Ionization in most systems of biological interest has an 

 important characteristic in respect to which it differs strikingly from 

 ionization in a gaseous system : because the freshly formed ions are quickly 

 hydrated by the ambient water (Sect. 4-4) and this hydration liberates a 

 great amount of heat, most of the energy transferred to the electronic 

 system of the molecule via the ionization act is dissipated almost imme- 

 diately after that act. The actual recombination is one of electrolytic 

 ions; it involves relatively small energy change; and it occurs in general 

 long afterward. An illustration of the profound influence which hydra- 

 tion may have on the mechanism of a radiobiological effect is provided by 

 the proposed mechanism for the inactivation of proteins to be described 

 below. 



It is to be expected that, in general, more than one, and very likely a 

 rather great number of the elementary processes mentioned above may 

 operate to cause observable change in a particular biological function. 

 For example, a number of different excited states may have nonzero, and 

 different, probabilities for nonreversed cleavage. The experimental 

 results should then follow the familiar manifestations of a "hit" phenom- 



