260 LOCALIZATION OF RADIATION EFFECTS 



stitution of radioactive carbon (C^'*) in the carboxy-peptide linkage of 

 protein. But, if we are to achieve a sufficient number of bond dis- 

 turbances from the disintegration act, it is more practical to use a rel- 

 atively short-lived isotope, like P^^. Such a short-lived isotope may be 

 used to study the effects of disturbing — P bonds in phosphorus-con- 

 taining structures, particularly in nucleoprotein. To exemplify this ap- 

 proach and the manner in which interpretation of results obtained await 

 the solution of purely physical problems, we will consider briefly some 

 of the recent experiments by Hershey, Kennedy, Gest, and the writer.* 



When bacteria and bacteriophage are grown in a medium containing 

 phosphate with a high P^^ content (0.003-0.03 per cent), the viral 

 progeny are unstable and show a progressive loss of infectivity with time. 

 We find that this inactivation is primarily the result of the radioactive 

 decay of the assimilated P, and not of the ionization resulting from pas- 

 sage of the beta particles through the phage. There is a linear relation 

 between the inverse of the P^^ content for phage and the average sur- 

 vival time. This relation holds over a sufficient range to provide a use- 

 ful technique for examining distribution of P^^ atoms among the phage 

 population. 



The phage particles studied may be thought of as spherical nucleo- 

 protein macromolecules with a maximal diameter of about 110 milli- 

 microns. Each particle contains '^5 X 10^ P atoms distributed in some 

 unknown manner. Some of these P atoms are radioactive and decay 

 with frequency determined by the disintegration constant, X. The re- 

 sults of this act in any given decay will be excitation and probably rup- 

 ture of the — P bonds, which may initiate a chain of reactions leading 

 to inactivation of the whole molecule. 



In analyzing the expected dependence of the phage survival on the 

 P^^ content, we begin by assuming (a) that disintegration of a single 

 atom can occasionally inactivate a phage molecule, (h) that all the phage 

 particles are equally radiosensitive, and (c) that the P^^ atoms are dis- 

 tributed at random among the phage particles. From these assump- 

 tions, it follows that the change of phage titer with time, —dS/dt, is 

 proportional to the number of disintegrations occurring in unit time, 

 which can be shown to equal 3.4 X 10^^a\NSAoe'''''K In this expression 

 the known quantities are X, the disintegration constant, or fractional 

 decay per day; N, the total number of P atoms per phage particle; Aq, the 



* A detailed presentation of the data and methods involved has appeared since 

 preparation of this article; see A. D. Hershey, J. W. Kennedy, H. Gest, and M. D. 

 Kamen, J. Gen. Physiol., 34: 305, 1951. The writer wishes to express his gratitude 

 to Dr. Hershey for his wiUingness to permit this discussion to appear before publi- 

 cation of the formal report. 



