SIZE, SHAPE, AND HYDRATION OF VIRUSES 49 



ject to systematic error and that several factoi-s need to be 

 considered before specific volumes are deduced. They conclude 

 that rather less water is associated with the virus, giving the 

 value 0.45 gm water per gram of virus. In consequence, the virus 

 diameter is also smaller, being 280 A. 



It must be agreed that, used in this way, virus motion studies 

 are highly informative. 



As a third illustration of virus motion studies we can consider 

 tobacco mosaic virus, as described in papers by Lauffer (1944) 

 and Schachman and Lauflfer (1949). All the beauty and sim- 

 plicity of the study of spherical particles are here absent. 

 Tobacco mosaic virus, although it can be thoroughly purified, 

 has a long rod structure which readily aggregates into multiple 

 lengths. Thus sedimentation, diffusion, and viscosity measure- 

 ments are all subject to the question of whether the virus has 

 aggregated or not. Nevertheless, Lauffer carried out sedimenta- 

 tion measurements on the preparation showing the least vis- 

 cosity. In the course of these measurements he was able to show 

 that the dependence of sedimentation constant on concentration 

 is due to a single factor, the viscosity of the solution in which the 

 virus is moving. In Fig. 2.7 is shown a plot of Lauffer's data 

 before and after the correction for the solution viscosity. The 

 apparent sedimentation constant is plotted as a function of virus 

 concentration, and it can be seen that the use of solution vis- 

 cosity (which is changing markedly with tobacco mosaic con- 

 centration) in place of solvent viscosity produces a reasonably 

 constant set of values. The value found for S20 was 187 S. This 

 treatment was also found to be valid for SBMV by Miller and 

 Price. 



For this same preparation, the diffusion constant was found 

 to be 2.62 X 10"^ cm^/sec, and the unhydrated partial specific 

 volume to be 0.73 ml/gm. Using these figures, a particle mass of 

 5.24 X 10~^^ gm, or a particle molecular weight of 31.6 X 10'^, 

 can be calculated. There now arises the same question of axial 

 ratio and hydration. Specific viscosity measurements combined 

 with sedimentation measurements yield an axial ratio of about 

 20. However, hydration must play a part. To establish this, 



