A Physical Mechanism for the Inactivation of Proteins by Ionizing Radiation 267 



secondary bonds (hydrogen bonds of various sorts, salt linkages, etc.) upon 

 which in large measure maintenance of the structure of the macromolecule 

 depends. The polarization corresponding to this absorption is established 

 during the period of from roughly 10^^ to 10^^ sec following charge localiza- 

 tion. Information concerning the magnitude of the dielectric constant sub- 

 sequent to such polarization apparently is lacking, but fortunately, according to 

 equation (1), the exact value is comparatively unimportant provided that it is 

 much greater than unity, which is certainly the case. (For water it is approxi- 

 mately five.) Thus an energy of about 35 kcal/mole is released during this 

 stage of polarization. It would be erroneous, however, to suppose that this 

 amount comprises the total effect. The Born energy of polarization is the 

 electrostatic energy difference between unpolarized and polarized states (free 

 energy), and this net diminution in energy includes a positive energy of the 

 various degrees of freedom (active coordinates of secondary bonds) equal in 

 magnitude to this net energy. Thus about 35 kcal/mole are dissipated to the 

 medium and 35 kcal/mole reside in the 'bonds' as potential energy resulting 

 from deformation and cleavage. A maximum of about fourteen secondary 

 linkages may therefore be broken. 



4. Orientation Polarization — This is the type of process usually considered in 

 studies (9) of dielectric relaxation of proteins (and, regrettably, often imagined 

 to be the only variety of dielectric absorption). It occurs at far greater wave- 

 lengths than the preceding types (e.g. at relaxation times of order of magnitude 

 10"'^ sec) and is without influence on the secondary bonds. (Thus, these electric 

 waves do not denature proteins, whereas intense irradiation in the 20 to 50-/^ 

 region would very likely do so.) 



To summarize, energy sufficient to dissociate about sixteen secondary linkages 

 will be released within an extremely short time interval after localization of 

 an electric charge of magnitude e in a protein molecule. Not all of this energy 

 need be used in bond rupture: a portion will be communicated to heat— for 

 example, to quantum oscillations, both primary and secondary, and also to 

 waves of long wavelength. But since the major interaction is with the secondary- 

 bond degrees of freedom, it is likely that the actual number of broken bonds is a 

 substantial fraction of the maximum number. A conservative estimate would be 

 ten. 



It is obvious that this consequence of ionization* will have a profound 

 influence on the structure. It is now universally believed that, as first proposed 

 by MiRSKY and Pauling (10), the organization of the macromolecules is 

 achieved and sustained by a very great number of secondary bonds, and that 

 the primary-bond structure is identical in native and denatured states. Modern 

 elaborations have refined details while retaining the basic ideas of Mirsky and 

 Pauling. (Thus Lumry and Eyring (11) distinguish between several different 

 arrangements of secondary bonds — for example, in the states of reversible and 

 irreversible denaturation — and propose the useful term conformation changes 

 for these variations.) In some respects denaturation may be viewed as a quasi 

 phase-transition, on a submacroscopic level. Although isolated secondary bonds 

 are continually being opened in random fashion by thermal energy, each bond 



* The essential idea was described briefly in a previous publication (5). 



