DISCUSSION ' 137 



apparent target size may differ from the size of the biological entity even in the 

 dry state. 



Morrison : 



I do not believe that ion migration in the organic molecule is a very likely 

 event. 



Burton : 



It is the electron migration, and not the ion migration, that is important. The 

 electron migrates; and, when it is thermalized, if it is thermalized near the water, 

 it is much more likely to be captured by water than by an organic molecule or 

 by a positive ion. 



Morrison : 



Is there any available estimate of the probability of electron escape without 

 damage to the organic molecule? 



Burton : 



Not that I know of. 



Applet ARD : 



I do not wish to defend a target theory necessarily as a main mechanism of 

 radiobiological action. It can, however, certainly be of use as an experimental 

 procedure. Pollard's group at Yale has tried to work under experimental con- 

 ditions which give such a target theory an optimal chance of apphcation, namely, 

 irradiation of dried materials with densely ionizing particles. Under these con- 

 ditions, for both enzymes and viruses, "operational" target sizes are found 

 which, while often less than the known sizes of these materials, appear to mean 

 something in the sense of defining a region with a high degree of molecular or- 

 ganization. 



Burton : 



I should have referred to the work of Pollard's group at Yale. Did not some 

 of the experimental evidence indicate a target size somewhat larger than the 

 geometrical size of the biological unit in question? 



Appleyard : 



No. To the best of my knowledge Pollard and Forro have never found target 

 sizes for phage larger than the size of the phage as evaluated by other methods. 



Burton (Communicated) : 



My question was founded on a misinterpretation of some data in the literature. 

 However, I should like to re-emphasize a point which should not be forgotten: 

 when dry biological material is irradiated the biological particle and the target 

 of target theory are identical. Any size discrepancy between calculated and 

 geometrical target is a reflection of inertness of a portion of the particle. 



