THE PHYSICAL PROPERTIES OF INFECTIVE PARTICLES 303 



The similarity (if it exists) is then in terras of effective density. Epstein and 

 Lauffer found that, by using indicator particles and sucrose solutions, the 

 infectivity of SBMV was surely associated with a class of particle of sedi- 

 mentation coefficient between 115 and 132 S, and that their results were 

 wholly consistent with the conclusion that the characteristic particle of 

 SBMV {s = 115S) was the infective one. 



By use of indicator particles it is possible to determine precisely the 

 sedimentation coefficient of an infective agent even in impure suspensions 

 where no "characteristic" particle can be found that will form a boundary. 

 The indicator particles are added in concentrations such as to form boun- 

 daries and their passage through the partition noted. By assay methods the 

 passage of the virus under study is also noted. If satisfactory indicator par- 

 ticles are available, the sedimenting characteristics of the virus can be 

 closely bracketed. 



Identification of virus particles by electron microscopy has these par- 

 ticular merits: the shapes and sizes of the particles can be determined quite 

 directly; extremely small quantities of material are sufficient for examination; 

 since particle counts are made, the specific infectivity of a virus preparation 

 can be measured; a rapid qualitative and quantitative assessment of im- 

 purity particles can be made. The advantages of the sedimentation method 

 are: the same aliquot of virus-containing material is both physically character- 

 ized and biologicallyassayed;itis feasible to make measurements upon a quite 

 impure suspension, particularly with the aid of indicator particles; an 

 identification of virus with particle can be made on considerations of effec- 

 tive density as well as on an indirect estimation of shape and size. It is 

 obvious that by employing both methods with at least partially purified 

 material there is little cause to doubt but what reliable physical identifica- 

 tions can now be made of many ty|)es of virus particles. 



C. Physical Properties of Well-Stndied Viruses 



1. Tobacco Mosaic Virus 



a. Introductio7i. Ever since its isolation by Stanley (1935), tobacco mosaic 

 virus (TMV) has been an object of interest to physical chemists exploring 

 the application of thermodynamic, hydrodynamic, and optical principles to 

 the study of macromolecules. Coupled with these investigations has been 

 the vigorous search into the structure and mode of action of TMV, perhaps 

 the best known of all of the viruses. As a result of the varied interests of the 

 many investigators, TMV has been subjected to more extensive research, 

 involving very diverse techniques, than has been focused on any other 

 macromolecuJe, naturally occurring or synthetic. 

 VOL. I — 21 



