A Physical Mechanism for the Inactivation of Proteins by Ionizing Radiation 271 



that interchain disulfide bonds may be sensitive regions for such an effect, 

 especially in view of the prominent contribution made by cystine absorption to 

 inactivation of proteins by ultraviolet light (21, 22), but there is no convincing 

 evidence that disulfide-bond cleavage is a major factor in protein denaturation 

 by ionizing radiation. Even the connection with ultraviolet inactivation is 

 ambiguous, because of the difTerent character of excitation produced by charged 

 particles (cf. infra). 



5. Ejject of the Environment on Radiation Sensitivity — Since the external 

 environment of the protein can and does participate in the structural stabiliza- 

 tion of the molecule, it may alter the effectiveness of the various possible dis- 

 turbances; the temperature effect already discussed is an instance of this. 

 For example, the medium can contribute externally and internally attached 

 water molecules, various interacting ions, and even chemical influences, and 

 the altered array of secondary bonds may clearly respond differently to the 

 disturbances caused by irradiation. After irradiation and resultant unfolding 

 the imposed forces may impede further unfolding and may, indeed, with the 

 help of thermal agitation, promote healing of the disorganization. On the other 

 hand they may under certain circumstances enhance the radiation sensitivity. 

 This accounts in a general way for pH and other solvent effects. There is 

 in principle no simple way to correlate such solvent influences with their effects 

 in ordinary thermal or biochemical inactivation, since the response to sudden 

 charge localization is completely different in character from that involved in 

 such phenomena. 



6. Spectrum of Radiation Injury — The previous considerations show clearly 

 that in a system of identical protein molecules exposed to any variety of ionizing 

 radiation, a broad range of effects on the molecules must occur. This variability 

 has its origin in (a), variations in the disturbance following localization of a 

 single charge, owing to both the intrinsic variability of the effect of the charge 

 at a given position, and to its localization at different possible sites (e.g. in 

 the interior or on the periphery of the molecule); and (b), in variations in the 

 cooperative effects discussed above, which can differ in number, degree, and 

 proximity (extent of overlapping of regions of charge-induced disorder is 

 obviously a cardinal factor). (Thus A'^^ certainly is not unique.) The consequence 

 is a wide range of change in properties, different molecules exhibiting qualitative 

 as well as quantitative differences. This spectrum of radiation injury is manifest 

 when appropriate measures are taken to detect it, and the suspicion arises that 

 the common conclusion from irradiation experiments that proteins are either 

 inactivated or unaffected cannot possibly be general, and may often be an 

 oversimplification or even an artifact imposed either by the conditions of an 

 experiment or its interpretation. That thermal denaturation is not a unique 

 transformation is, of course, elementary knowledge; on the basis of the present 

 analysis it appears likely that radiation denaturation may cover an even broader 

 range. Ample proof of the spectrum of radiation injury is provided by the work 

 of Fricke (23, 24). Thus partial unfolding of the main chains, in addition 

 to denaturation, is indicated by changes in optical rotation, serological response, 

 and A//:J:, and there is evidence for a small amount of fragmentation, with 

 formation of a variety of products of lower molecular weight. The so-called 

 'after-effect', a diminished thennal stability of irradiated proteins, is simply 



