60 THE PHYSICS OF VIRUSES 



which merely consists of water bound on the outside, or even 

 mechanically seeped into cracks in the structure, will not con- 

 tribute to the scattering. On the other hand, water, which causes 

 a swelling of the whole virus, will produce an effect because the 

 radius of gyration will be altered. Thus X-ray scattering offers 

 a new measurement of size and shape, with a different response 

 to water associated with the virus. More detailed information 

 regarding hydration can thus be obtained for a virus which has 

 been studied by sedimentation, diffusion, and viscosity, as well 

 as by X-ray scattering. 



The price that has to be paid is the need for very pure prepara- 

 tions of high concentration, concentrations of the order of 1% 

 are needed, and any considerable amount of virus impurity is 

 serious. For this reason, attractive studies of serological precipi- 

 tates are not so easy to carry out. 



The small-angle scattering alone yields the value of R, the 

 radius of gyration. If the molecular weight is independently 

 known, it is possible to calculate the axial ratio for an assumed 

 ellipsoid. This axial ratio is for the hydration-free molecule 

 since the effect of water is deducted from the virus. By compar- 

 ing the X-ray figures with axial ratios derived from the frictional 

 ratio in sedimentation, which corresponds to the hydrated 

 molecule, the degree of hydration can be inferred. 



A study of the two spherical viruses, southern bean mosaic and 

 tobacco necrosis, by Leonard, Anderegg, Kaesberg, Schulman, 

 and Beeman (1950, 1951) has enabled them to make measure- 

 ments of the radius of gyration in each case using only the small 

 angle scattering. The results are listed in Table 'iA. The inter- 

 ference radius of bushy stunt is included. 



