SIZE, SHAPE, AND HYDRATION OF VIRUSES 33 



The ratio ///o is thus 



/ _ ///••'(! - I'op) 



/o 



(iTr]S 



47r 



:^2.i()) 



Replacing m by the value found from combined sedimentation 

 and diffusion, we have, because in — kTS/D(l — Vop), 



These relations are all set out in Svedberg and Pedersen (1940, 

 Eqs. 68-70b). Actual values for the constants applicable to 

 20° C are given there. 



It can be seen that, by employing Eq. 2.17, the results of 

 sedimentation, diffusion, and partial-specific-volume measure- 

 ment can, in the case of nonspherical viruses, yield a value of 

 f/fo, the ratio of the frictional drag coefficient to the equivalent- 

 sphere frictional drag coefficient. 



One obvious reason why f/fo should not be unity is the fact 

 that the virus may have an asymmetrical shape. If this is known 

 to be not the case — the virus is known to be spherical — one 

 therefore predicts that the use of proper measurements together 

 with Eq. 2.17 should give f/fo a value of unity. This is not found, 

 and the explanation given in such a case is the presence of 

 hydration, water which is bound on the surface and in the virus 

 interstices so that the radius applicable to the frictional drag 

 is not the radius of the dry particle. For the case of diffusion 

 measurements, where the frictional drag term is all that is 

 important, the effect of r gm of water (partial specific volume, 

 T"i) combined with one of virus (partial specific volume, To) is 

 to produce a frictional drag, fi>, where 



fo _ jrW + Vo\'' ^^^^ 



/o [ Vo 



In the case of sedimentation, the buoyancy term is also of impor- 

 tance, so the ratio to be applied as a correction factor for sedi- 



