274 Robert Platzman and James Franck 



Another factor which diminishes the effectiveness of excitation by uhraviolet 

 light is the spatial isolation of the individual absorption events. Thus the second- 

 ary bonds ruptured as a consequence of a single internal conversion process may 

 heal before serious unfolding occurs. In the case of excitation by charged 

 particles, however, the excited molecules are often produced in close proximity 

 (and simultaneity) to other activations, and hence must undoubtedly contribute 

 to the disorganizing action. Such collective effects have already been discussed. 



One characteristic of ionizing radiation which always should be kept in 

 mind is the difference in nature of the excited molecules produced from those 

 that have been studied photochemically : they correspond, for the most part, to 

 radiation in the vacuum ultraviolet region, where most of the optical transition 

 probability invariably lies. Little is known of polyatomic molecules with regard to 

 optical phenomena and to processes following excitation in this spectral region. 

 However, such radiation may have far greater potency than the readily accessible 

 ultraviolet, for either in dissociation or in internal conversion processes, it always 

 releases sufficient kinetic energy to break many more secondary bonds. 

 There is indeed some experimental indication of this in the rise of quantum 

 yields at the shortest wavelengths studied (25, 22). Thus the role of excitation 

 in radiobiology probably is greater than usually is (cf., e.g., (14)) supposed. 



V. CONCLUSION 



The mechanism considered here provides a realistic physical basis for 

 understanding the remarkable fact that a polar macromolecule of molecular 

 weight as great as 10^ can be inactivated by only a few ionization acts, and it is 

 capable of explaining qualitatively a variety of experimental results. It replaces 

 the notion that ionizing radiation acts merely by breaking chemical bonds 

 directly, which, apart from its superficiality, does not actually explain denatura- 

 tion at all. No attempt has been made in the present paper to analyze in detail 

 the myriad data on numerous kinds of radiation effect for varying quality and 

 quantity of radiation, varying environment, etc. Indeed, further development 

 must await, in most points, the further elucidation of protein structure, especially 

 in its dependence upon the secondary-bond configuration. In particular, the 

 number, disposition, and mutual dynamical behavior of the secondary bonds, 

 as well as the character of their large-scale stabilizing action, must be more 

 fully understood. At some future stage of development radiation studies may 

 provide a valuable tool in advancing this knowledge, for the action of ionization, 

 as described here, is completely different in character from other types of 

 attack which are investigated, such as heat, salts and other chemically inert 

 solutes, and chemical agents, all of which act essentially adiabatically at the 

 atomic level. In essence it has been demonstrated that the marvellous stabilizing 

 action manifested in natural polar macromolecules is intrinsically ineffectual 

 against the nonadiabatic disturbance of an ionization act. 



REFERENCES 



1. D. E. Lea: Actions of Radiations on Living Cells, Cambridge University Press, Cambridge, 

 England (1946). 



2. L. H. Gray: The initiation and development of cellular damage by ionizing radiations. 

 Brit. J. Radiol. 26, 609-618 (1953). 



