SIZE, SHAPE, AND HYDRATION OF VIRUSES 47 



suitable for detailed study. The preparations used were not 

 crystalline but were purified by both sedimentation and chemical 

 methods. Both processes gave the same results. Sedimentation 

 measurements were made at various concentrations. The sedi- 

 mentation constant slowly fell as the concentration increased. 

 By plotting 1/S versus concentration and extrapolating the 

 resulting straight line to the value 1/S at zero concentration, 

 the sedimentation constant at 20° C was determined to be 115 S. 



Diffusion measurements were carried out in an electrophoresis 

 cell, with the result that the diffusion constant at 20° C was 

 found to be 13.4 X 10~^ cmVsec. Partial-specific-volume meas- 

 urements on the dry virus gave the value 0.696 ml/gm. Putting 

 these together, the particle molecular weight was found to be 

 6.63 X 10^ 



The individual sedimentation and diffusion measurements 

 were then applied to Eq. 2.17 for the frictional drag ratio, with 

 the result that a value of ///o = 1.25 was found. 



The viscosity of the virus was measured as a function of con- 

 centration, with the results shown in Fig. 2.6. The value of (f) 

 so found was 1.76 if the particles are assumed to be spherical and 

 the Einstein relation, 17/770 — 1 = 2.50, can be applied. Alter- 

 natively, if the virus is supposed to be unhydrated, and an axial 

 ratio is responsible for the large viscosity, the axial ratio deduced 

 is 5.5 to 1. 



In addition to these measurements, the sedimentation rate in 

 sucrose solutions of different density was measured. The sedi- 

 mentation rate did not vary linearly with density, probably due 

 to osmotic effects or combination between virus and sucrose. 

 However, by carefully determining the slope of the line at very 

 low sucrose concentrations and extending this to the point of 

 zero net force, the density value for the sucrose solution was 

 found to be 1.21, corresponding to a partial specific volume 

 (hydrated) of 0.827. The curvature of the line was actually 

 slight, though definite, and the data are so good that the above 

 figures should be very reliable. 



We now have the following experimental facts about the virus. 

 First, the particle molecular weight is 6.63 X 10*^. Second, 



