322 H. K. SCHACHMAN AND R, C. WILLIAMS 



that poliovirus is polyhedral in shape is not entirely ruled out, however, 

 because of the uncertainties associated with trying to distinguish between a 

 polyhedron and a sphere with a particle so small. 



Some hydrodynamic study of poliovirus has been made with highly 

 purified material and in dilutions sufficiently great (0.005 %) as to approxi- 

 mate infinite dilution. Sedimentation shows a single, sharp boundary with 

 an S2Q, u- = 158 S for all three strains (Schwerdt, 1957). An attempt has been 

 made to determine the partial specific volume by sedimentation in varying 

 mixtures of HgO — DgO and extrapolation to zero sedimentation velocity. 

 The reciprocal of the partial specific volume found is 1.58 gm./cc. (Schwerdt, 

 1957). 



In the absence of diffusion data the molecular weight of poliovirus carniot 

 be evaluated unambiguously from its sedimentation coefficient. An approxi- 

 mate value can be obtained, however, through the use of certain assump- 

 tions. If the sedimenting unit is assumed to be a uniform, solid sphere, with 

 no water of imbibition, the molecular weight that is calculable from the 

 sedimentation coefficient and particle specific volume is 5.4 X 10^, and the 

 particle diameter is 220A. Such a model is implausible if one generalizes 

 from other viruses, such as tomato bushy stunt and turnip yeUow mosaic. 

 Some imbibition of water is most likely when the poliovirus is in solution, 

 and this would cause the frictional ratio (///o) to be greater than 1.0. If the 

 value of ///o =1.15 generally found for proteins and viruses is used in the 

 calculations, the molecular weight turns out to be 6.7 X 10^. 



It is also possible to calculate the molecular weight of poliovirus from its 

 measured partial specific volume and its diameter determined electron 

 microscopically. This amounts to assuming that the 270A particle has an 

 average density of 1.58 gm./cc. The molecular weight is then calculated to 

 be 10 X 10«. 



It is obvious that the first and last calculations are likely to yield too low 

 and too high a molecular weight, respectively. It is milikely that the virus 

 sediments as a solid sphere, completely dry both inside and out. It is also 

 unlikely that the 270A frozen-dried particle seen by electron microscopy is 

 solid throughout and contains no empty spaces. A compromise way of calcu- 

 lating the molecular weight, in which both the sedimentation and the 

 electron microscope data are used, is to assume that the virus in solution 

 has a hydrated diameter corresponding to the electron microscopic diameter 

 of 270A diameter. This calculation yields a molecular weight of 6.8 X 10^. 

 It is evident that aU values of the molecular weight of this virus are on shaky 

 gromid, since the partial specific volume is known only inexactly, and the 

 diameter of the solvated particle is uncertainly estimated. The hydro- 

 dynamic data for poliovirus are at present limited in precision by the 

 availability of only small amounts of highly purified material. 



