76 ERNEST POLLARD 



Guild, Hutchinson and Setlow, 1955; Pollard, 1959) and the important 

 conclusions are as follows. 



1. An ionization within the volume of a protein molecule or a nucleic 

 acid molecule has a very high probability, in excess of 0-5, of re- 

 moving its biological action. The mechanism of this action in protein 

 is beginning to be understood and is probably related to the migra- 

 tion of excitation energy to an -S-S- linkage which thereby becomes 

 sensitized. In nucleic acid it is probably l)reakage of the chain, 

 or possibly cross-linkage. 



2. A nucleoprotein molecule, such as a small virus, can also be inacti- 

 vated by ionization. The probability of inactivation is less, but still 

 high. 



3. Estimates of the volume of biologically important molecules based 

 on the theory that for activity to remain after irradiation they 

 must have wholly escaped any ionization whatever, are very in- 

 formative and have proved to be correct within a factor of two, 

 with few exceptions. 



4. Within cells, the radicals formed l)y ionizing radiation do not diffuse 

 more than 30 A before they encounter some structure or molecule 

 which removes them. 



The evidence in support of the above four conclusions cannot be given 

 in this paper because time does not permit. Table I shows the results of 

 irradiation of several enzymes, with the conclusions drawn from the 

 statistics of complete escape. Table II shows some results of irradiating 

 viruses. Table III shows the results of experiments largely by Hutchin- 

 son (1960) on which the distance of 30 A has been estimated. It will be 

 useful later to see what conclusions can be drawn about protein 

 synthesis using the same analysis. 



Table I 



