50 



THE PHYSICS OF VIRUSES 



Schachman and Lauffer made sedimentation rate measurements 

 in both serum albumin and sucrose solutions, with the results 

 shown in Fig. 2.8. It can be seen that the no-net-force point 

 occurs for a solvent density of 1.127 in the first case and 1.266 in 

 the second. These do not agree. The first requires hydration to 

 the extent of 66% by volume per milliliter of virus, and the 



1 80S 



I50S 



I40S - 



I20S 



1.0 

 Concentration 



2.0 



ViOOcc) 



Fig. 2.7. Sedimentation constant of TMV versus concentration. The vis- 

 cosity of the solution increases as the concentration increases and, if allowance 

 is made for this, the dotted line results. Data and analysis of Lauffer (1944). 



second to 27%. Following a suggestion made by Kauzmann, they 

 propose that a layer of water roughly half the thickness of the 

 solute molecules is entrained by the virus. Correcting for this, 

 they conclude that the hydration of tobacco mosaic virus is 15% 

 of the wet particle, or about 0.12 gm water per gram virus. It is 

 likely that the hydration of SBMV is less if this type of correc- 

 tion is made. 



With this figure, the sedimentation, diffusion, and viscosity 

 data fit with a rod-shaped virus of length 2,500 A and width 



