270 Robert Platzman and James Franck 



the analysis of spatial correlations of charge production* (also called 'spur' 

 or 'cluster' distribution), a subject that has not yet been brought to a 

 quantitative basis, j (It should be emphasized that the bond ruptures caused 

 by positive and negative charges arising from a single spur are all essentially 

 simultaneous.) 



If accurate information concerning these correlations were available, the 

 path would be open for study of A^^-, and it could be anticipated that both N^ 

 and the ratio NJN would prove helpful in the study of protein structure. 

 A closely related subject is the dependence of denaturation efficiency on the 

 so-called density of ionization; the mechanism predicts that as this parameter 

 increases, the effectiveness first rises (as more of the charges are formed in 

 proximity to one another) but ultimately declines, on an energy basis (as the 

 number of bonds ruptured in a molecule exceeds the minimum number required 

 for unfolding). This is indeed observed in many types of experiment, although 

 the rising or the declining portion of the dependence may be enhanced or 

 suppressed in individual cases, depending upon the specific effect. Such 

 behavior should be distinguished from the corresponding one of simple radia- 

 tion-chemical systems (18, 19), which stem from secondary chemical reactions 

 occurring subsequent to the primary processes; the distinction is not trivial. 

 (In the case of complex biological systems, such as whole cells, the dependence, 

 although it often appears to be similar and may be closely related, must clearly 

 have a far more complex origin (20).) It may be noted that for large protein 

 molecules the disorganization about even a densely ionized track may be 

 insufficiently extensive to produce denaturation. Hence there would be antici- 

 pated some theiTnal sensitivity, although in general less than in the case of 

 sparsely ionizing radiations. Combination of the disorder produced by several 

 localized electric charges is by no means the only possible kind of collective 

 effect, for contributions may be made by excitation events and even by energy 

 transfer to valence-bond and secondary-bond oscillations from subexcitation 

 electrons, both of which must by themselves be minor influences. Changes in 

 certain molecular properties may indeed demand such collective action. For 

 example, permanent dissociation of a valence bond following an excitation act 

 is very unlikely in proteins, but if a dissociative excitation and an ionization 

 should occur close together, the secondary-bond breakage caused by the 

 ionization would prevent heahng of the rupture. Subsequent thermal action 

 would then denature and fragment the molecule. It is a suggestive possibility 



* Following Lea (1), many investigators have inferred from their experimental data that 

 inactivation is accomplished by a single 'average' primary ionization. This is in rough agree- 

 ment with the general conclusion reached above, but it is not a quantitative statement. Most 

 analyses of experimental data currently being offered appear to be insufficiently detailed and 

 accurate to refine it. 



t The proposal that Auger cascades may have an important role in the chemical and 

 biological effectiveness of ionizing radiation (17) is highly relevant to the conclusion that a 

 single electronic charge will in general be subcritical, for each cascade must unquestionably 

 result in destruction of the secondary-bond structure on an extensive scale. (One factor is 

 shown by equation (1): the polarization energy is proportional to the square of the electric 

 charge.) In this connection it may be mentioned that the detailed calculations in the paper 

 cited apply only to heavy-particle irradiation; for fast electrons and gamma-rays the yield of 

 Auger cascades is very much greater, being of the order of magnitude of a few per cent of all 

 ionization events, in proteins. 



