A Physical Mechanism for the Inactivation of Proteins by Ionizing Radiation 269 



except in the case of the smaller proteins, for which the energy and entropy 

 requirements probably do not correspond well with the averages assumed. 

 However, an entropy increase of 12 cal/mole deg is much greater than would 

 be expected for simple cleavage of a secondary bond. This suggests the entirely 

 reasonable conclusion that unfolding occurs when there are broken, not any N 

 secondary bonds, but a particular selection of A^ of them. Clearly, the selection 

 must be a very special one, embracing bonds at certain decisive locations. 

 (Because of cooperative effects this would be true even if all of the bonds were 

 equivalent in their stabilizing action, which is unlikely to be the case.) In the 

 case of secondary-bond rupture following ionization, the bonds aff"ected are 

 more or less localized, and therefore less effective, on the average, than the 

 numbers A'^ listed in the table. Hence the required number of ruptured bonds 

 for ionizing radiation, TV,, must substantially exceed A'^. Since A^ is in the 

 neighborhood of ten for even the smallest enzyme molecules, it is evident that 

 the effect of a single electronic charge is almost, but not quite violent enough 

 to inactivate a typical, small protein macromolecule. Even the combined 

 eff"ects of the positive and negative charges, if they are localized in the same 

 molecule, which must usually be the case, may be expected to be just 'subcritical' 

 (except, possibly, in the case of the smallest molecules). 



This conclusion leads immediately to the following important consequences. 



1 . Variation in the Response of Various Proteins — Because of differences in 

 structural features among proteins of comparable size, the effectiveness of one 

 or two charges may have wide variation. Furthermore, Ni would be expected 

 to increase with the molecular volume, but not necessarily in a simple way. 

 (Note that A^ in Table I shows a definite correlation with molecular weight 

 ( IV), but that W/N is by no means constant.) In all hkelihood NJN would also 

 exhibit interesting differences. 



2. Effect of Temperature on Radiation Sensitivity — ^In cases in which a 

 single electric charge (or a pair of them) is subcritical, its effect may be critical 

 at elevated temperatures, because of the augmented probability that the ambient 

 thermal disorder can supplement the radiation eflTect and bring it past the 

 threshold for denaturation. This explains, qualitatively, the pronounced thennal 

 sensitivity which has been observed for some inactivation cross sections (14, 

 15) and which apparently has not received a satisfactory interpretation here- 

 tofore (14). 



3. Effect of Anisotropy on Radiation Sensitivity — ^Anisotropy of the structure 

 (at the microscopic level) may contribute greatly to the radiation sensitivity. 

 An extreme example would be DNA, which is stabilized by numerous secondary 

 bonds having a degree of freedom for oscillation with an almost common 

 direction. Abrupt production of electric charge would rupture many of these 

 bonds simultaneously, causing a portion of the structure to collapse. It is entirely 

 possible that the great radiation sensitivity of DNA, which is found in a variety 

 of experiments (16), may have its origin, at least to some extent, in this effect. 

 The predicted role of molecular anisotropy might be tested experimentally, since 

 proteins and allied molecules exhibit interesting differences in this characteristic. 



4. Collective Effect of Individual Activations on Radiation Sensitivity — 

 Although a single electric charge may be insufficient to eff'ect denaturation, a 

 very small number of them would suffice. This points to the importance of 



