SIZE, SHAPE, AND HYDRATION OF VIRUSES 45 



particle is being treated. Thus, again, an ambiguity between 

 hydration and axial ratio exists. 



What is done is to assume a reasonable hydration and axial 

 ratio to fit diffusion data and then to see whether a check against 

 viscosity is obtained. Actually, there is no virus for which the 

 whole elaborate technique of protein molecular measurement 

 has been applied. Very nearly, it has been done for tobacco 

 mosaic virus and southern bean mosaic virus, with results to 

 be described in what follows. 



Examples of the Use of Virus Motion Studies 



Some examples of virus motion studies can be given here. 

 These are by no means the only cases studied, but are chosen as 

 representative of the kind of work that can be done. The first 

 is the study of potato yellow dwarf virus by Brakke, Black, and 

 Wyckoff (1951). The i)urification of this virus to a high degree 

 is difficult, as it is not very stable and centrifugation has to be 

 carried out near the freezing point. Nevertheless, concentrated 

 preparations of high infect ivity could be obtained, and these 

 were centrifuged in horizontal capillaries. During the cen- 

 trifugation a quite definite visible boundary developed, and it 

 was possible to measure the sedimentation constant of this 

 boundary. In addition, virus samples were removed by a fine 

 capillary, and the sedimentation constant of the infectivity was 

 determined. The constant varied with concentration. This was 

 traced to the fact that the viscosity of the solution increased with 

 concentration. By plotting the apparent sedimentation con- 

 stant versus concentration, a reasonably good straight line was 

 obtained which was extrapolated back to zero concentration to 

 give a value of 1,150 S for water at 20° C. 



The variation of sedimentation rate with solvent density was 

 plotted, using sucrose as the second solute. The results extra- 

 polate to a zero sedimentation rate at a density of 1.17, so the 

 hydrated density of the virus is 1.17. This may need correction 

 for the osmotic effects of the small second solute molecule. 



The authors examined, with some care, several high concen- 

 tration preparations of potato yellow dwarf virus in the electron 



