250 RADIATION BIOLOGY 



action can cause a particular effect. Thus, an enzyme might be destroyed 

 either by denaturation of its protein component, or by decomposition of 

 its influential prosthetic group.) In the following paragraphs a proposed 

 mechanism (Franck and Platzman, to be published) for the denaturation 

 of proteins effected by high-energy radiation is outlined briefly. Whether 

 or not it should prove to be correct, it illustrates a type of analysis which 

 is capable of explaining nonlocalized action. And above all, it introduces 

 a factor which has heretofore been grossly neglected in considerations 

 of basic mechanisms in radiobiology, but which is likely to be of cardi- 

 nal importance, namely, the time dependence of electrical forces arising 

 from dielectric dispersion. 



A single protein is an intricate complex consisting of many polypeptide 

 threads bonded together in a highly ordered structure by man}^ compar- 

 atively weak hydrogen bonds. These bonds have dissociation energies of 

 about 5 kcal per mole (about 0.2 ev). Denaturation of the protein, 

 according to Pauling, is the result of breaking enough of these bonds in 

 one area that much of the order is destroyed ; the destruction is irreversible 

 if the bonds are all broken at about the same time, because there is then 

 negligible probability that when they reform, the threads will arrange 

 with the same organization. Although the dissociation energy of any one 

 bond is sufficiently small that it will often be broken and "healed" again 

 at room temperature, there are so many of them available to maintain the 

 high order by forcing an individual healing to conform to that order that 

 an appreciable number must be broken at almost the same place and time 

 if the order is to be permanently destroyed. It is possible in this way to 

 account for the relatively sharply defined, elevated temperature required 

 for thermal inactivation. 



A single electronic excitation act occurring in one of the amino acids of 

 the protein will almost immediately be followed by internal conversion 

 (amino acids are nonfluorescent), thus activating the oscillations in the 

 molecule to the extent of about 10^ oscillational quanta. Since only two 

 or three such quanta need be concentrated in a single degree of freedom of 

 a hydrogen bond in order to break that bond, it is likely that as the oscil- 

 lational quanta are dissipated throughout the original amino acid mol- 

 ecule and thence to others, about 10 to 20 hydrogen bonds may be broken. 

 (The numerical estimates are meant to suggest orders of magnitude only.) 



A single ionization act influences the bonding in quite a different man- 

 ner. As mentioned above, most of the energy thereby delivered to a 

 polar medium is dissipated to heat almost at once. The positive ion will 

 become hydrated, and in so doing will cause roughly 10 to 20 water mol- 

 ecules to reorient in its Coulomb field within a period roughly equal to the 

 dielectric relaxation time, which is approximately equal to that of pure 

 water, 10^^^ second. [The Coulomb field is much stronger during this 

 period than is that about an electrolytic (hydrated) ion, for the effective 



