PHYSICAL PRINCIPLES OF CHEMICAL REACTIONS 251 



dielectric constant is not the static one, namely about 80, but rather an 

 average, during the time interval concerned, of the "high-frequency" 

 dielectric constant; this average value will be about 3 to 5.] In simple 

 terms, the newly created ion functions as a powerful drying agent. This 

 reorientation of the water molecules will cause the breaking of approxi- 

 mately twenty hydrogen bonds. 



The effect of penetration of the protein by a swiftly moving electron 

 is now considered. Many calculations in the literature, based on the 

 value of the mean energy per ion pair expended by high-energy radiations 

 in ionizing gases (about 32 ev), have often been taken to indicate that a 

 single ionization act in a protein can cause its inactivation. Because the 

 mean energy per ion pair is probably about the same in a protein medium 

 as in pure water, and therefore closer to one-half the above-mentioned 

 value (Sect. 4-4), the minimum effective action is very likely at least two 

 ionizations, and with these there will always be associated about two to 

 four excitations. The electron liberated in the ionization act will also be 

 capable of damaging the structure. If, as must usually be the case, it is 

 captured within the protein to form a negative ion, or if an electron from 

 an external ionization is captured within the protein, the hydration of 

 that ion will contribute another twenty or so bond ruptures. (Moreover, 

 the electron itself is ejected with a kinetic energy which can vary within 

 wide limits but is on the average a few ev, and this energy will be trans- 

 ferred to atomic vibrations as the electron penetrates the protein. Fur- 

 ther damage may be caused by chemical reactions of radicals formed 

 internally in the protein by dissociation of the aforementioned ions. 

 Even the effect of the penetrating particle on that portion of the environ- 

 ment adjacent to the protein will contribute, for H and OH radicals 

 formed within some 10 to 20 A may diffuse to the protein surface and 

 cause further disruption). Thus, a minimum of 10" hydrogen bonds will 

 be broken when an electron traverses a protein, and in most traversals 

 even more breakages will occvn*. These breakages will be located chiefly 

 within a radius of approximately 10 A, and will be virtually simultaneous 

 in time: the disturbances will overlap to such an extent that entire folds 

 will be unraveled, and will be unable to re-form properly. 



It is obvious that the damage sustained by a protein from penetration 

 of a densely ionizing particle like an a particle will be so much greater than 

 in the case of electron penetration that inactivation must be inevitable. 

 Indeed, a simple calculation shows that, since most of the energy trans- 

 ferred from the a particle to the medium is immediately dissipated to 

 heat (for excitations, by internal conversion; and for ionizations, by 

 hydration), the heating effect, even with due allowance for heat loss to 

 the surroundings, must bring the column about the a-particle track to a 

 transient temperature of at least 100°C, and that this must persist for at 

 least 10~" second and possibly much longer. The destructive effect of 



