SIZE, SHAPE, AND HYDRATION OF VIRUSES 



:u 



The measurement of the sedimentation constant alone thus 

 gives the ratio w(l — Vop)/f. 



Particle Weight Measurement 



The combination of diffusion and sedimentation offers a 

 method of measuring the virus mass directly if the partial 

 specific volume, Vo, is known. For we can combine the relations 



m{l — Vop) 



D = kT/f 

 to eliminate / and give 



and 



S = 



/ 



kTS 



VI = 



D{1 - VoP) 



(^2.13) 



In this expression, the buoyant term, — ]^op, corresponds to 

 the reverse force due to the displaced water. Therefore, in 

 principle. To should be the partial specific volume of the dry 

 virus, and m will then be the mass of the corresponding dry 

 virus. The value of T^o is accordingly found by measuring the 

 increase in the volume, dv, of the virus solution due to the addi- 

 tion of a mass dm of virus. Vq is then dv/dm. Using values 

 obtained in this way, the results of Table 2.3 have been obtained. 



TABLE 2.3 

 Masses of Viruses Determined by Sedimentation and Diffusion 



Virus 



Equivalent* 

 molecular 

 Mass (gm) weight 



Reference 



Southern bean mosaic 1.09 XIO"!^ 6,600,000 Miller and Price (1946) 



Bushy stunt 



Rabbit papilloma 

 Tobacco mosaic 



1.7 X 10-1^ 10,300,000 Lauffer and Stanley 



(1940); Neurath and 

 Cooper (1940); Mc- 

 Farlane and Kekwick 

 (1938) 

 7.0X10-16 47,000,000 Neurath ei a/. (1941) 

 5.0 X 10-16 30,000.000 Lauffer (1944) 



* The equivalent molecular weight is the product of the particle mass and Avogadro's 

 number, 6.03 X 10^'. This is more familiar to many, though it is doubtful if much is 

 gained by thinking of a virus as a molecule. 



