138 ELEMENTARY CHEMICAL PROCESSES 



Abelson: 



Since the hit theory has come under discussion, I wish to point out a case in 

 which the single-hit target theory seems to be relatively meaningless. Roberts 

 at the Carnegie Institution of Washington has shown that small differences in 

 biological manipulation can give large changes in the apparent multipUcity of 

 hits. By small variations in time of incubation in a sahne solution after ir- 

 radiation, either single-hit or multiple-hit curves may be obtained for the sur- 

 vival of E. coll irradiated by ultraviolet light. 



Burton: 



The target theory in its original form identifies a hit with an ionization. The 

 target theory does not demand that a single hit be effective. A single hit has 

 been found, by analysis of empirical results, to be effective in certain cases 

 (for example, viruses and chromosomes), but multiple hits may be required in 

 other cases. 



Hart: 



Oxygen molecules play an important role in the radiolysis by x-rays of aqueous 

 solutions of formic acid and hydrogen peroxide. In the absence of oxygen, a 

 chain oxidation of formic acid takes place, resulting in the overall reaction: 



HCOOH + H2O2 = 2H2O + CO2 



Oxygen is an excellent inhibitor for tliis reaction. The hydrogen atoms pro- 

 duced during irradiation react with oxygen in preference to formic acid even 

 under conditions where the ratio of formic acid to oxygen is 10,000 to 1. In the 

 absence of oxygen and hydrogen peroxide, the hydrogen atoms do, however, 

 react with formic acid to produce molecular hydrogen. This has been demon- 

 strated by the irradiation of DCOOH in aqueous solution. HD is a primary 

 product of this reaction. Therefore it is apparent that hydrogen atoms do not 

 require oxygen in order to promote chemical changes in solute molecules. 



Burton (Communicated): 



I did not intend to imply, in my paper, that hydrogen atoms require oxygen 

 in order to be effective. As a matter of fact, we can guess that the activation 

 energies of processes (in biological systems) invohing atomic hydrogen will, in 

 general, be lower than those of processes involving HO2. This fact is precisely 

 why HO2 survives longer and diffuses farther in such systems, and serves to 

 explain the role of oxygen in processes in which the hit may not be in or near the 

 target. In the interesting case described by Hart the atomic hydrogen reacts 

 with formic acid whenever it is not removed by some other process. 



