A Physical Mechanism for the Inactivation of Proteins by Ionizing Radiation 265 



energy coinmunicated in conjunction with the ionization act in the form oi' 

 electronic excitation energy of the parent ion. In the vapor phase this additional 

 energy may have time to be concentrated in a particular degree of freedom, but in 

 condensed phases it will ordinarily be dissipated by internal conversion and 

 thermal conduction, leaving the parent ion in its ground electronic state, which 

 is usually stable. Hence the simple identification of an ionization act with 

 splitting of a molecule, which is so common in the literature of radiation 

 chemistry and radiobiology, must be viewed with skepticism in so far as con- 

 densed phases are concerned. Such rupture may indeed occur, however, during 

 the growth of polarization about a freshly formed electric charge. It is then 

 very much a consequence of the interaction of the ion with its environment, 

 and in the case of valence-bond breakage imposes special energy requirements 

 with regard to both availability and mode of communication. These require- 

 ments may be satisfied, for example, in instances where dissociation would 

 lead to much greater localization of the charge, and therefore to greater polariza- 

 tion energy. 



An important property of the energy transferred to the medium by virtue of 

 the growing polarization about a freshly produced electric charge is that most 

 of it is transferred in initial stages, during which the electric field strength is 

 very great. This follows from the Born formula 



^E ^ (^2/2/?)[(l/e(r,)) - (l/e(?2))] (1) 



giving the difference in self-energy of the electric field about a charge of magnitude 

 e outside the sphere of radius R, between instants when the eff'ective dielectric 

 constant has the values ^{t-^ and €{t<^. Since the electronic polarization 

 (oj/-^ 10^^ sec~^) is effective virtually instantaneously, the initial value of e is 

 approximately equal to the square of the ordinary refractive index n, or about 1 .5. 

 By the time that e has increased to (say) 5, most of the total energy 

 (e^l2R){lln^ — 1/eJ will have been dissipated. (Paradoxically, if e^,^ 1 the 

 behavior under discussion is nearly independent of the magnitude of e^.) This 

 argument has the important consequence that a major portion of energy trans- 

 ferred to a polar medium by virtue of an ionization act will be communicated, 

 in a very short time, to degrees of freedom associated with weak polar 

 ('secondary') bonds, and that the region receiving this energy will be considerably 

 more extensive than that affected by a slow change in electric field intensity. The 

 total energy so communicated will be of the order of magnitude of 100 kcal/mole 

 for each (electronic) charge produced, but will depend upon the 'size' of the 

 positive ion, or, in general, upon the structure at a molecular level. If the bonds 

 aflTected are very weak, a substantial fraction of them will be broken, so that the 

 corresponding amount of energy cannot properly be said to have been converted 

 to heat: a portion of it truly has been used to 'melt' a certain structure; 

 obviously, subsequent 'resolidification' often will occur and then will release 

 part or all of the energy as heat. 



The fate of the ejected electron is similar. It will progress a distance more 

 than sufficient for the developing polarization to inhibit initial recombination 

 (6), and eventually will attach itself to a negative-ion forming group such as 

 OH, if one is available, or to some other entity capable of binding it. The net 



