SIZE, SHAPE, AND HYDRATION OF VIRUSES 53 



measurements, LaufFer and Stanley conclude that the particle 

 diameter is about 1,000 A and the degree of hydration is 0.60 gm 

 of water per gram of virus. Lauffer and Taylor (1953), with more 

 modern buoyancy observations, find the sedimentation constant 

 for the virus to be slightly higher and, from the two sets of data, 

 conclude that the hydration is 0.32 gm water per gram of virus. 



It is hardly possible to tabulate here all the observations on 

 virus motion studies. Bawden (1950, p. 2*24) tabulates the 

 sedimentation constants of 11 plant viruses, and Lauffer, Price, 

 and Petre (1949, p. 188) tabulate sedimentation constants for a 

 variety of viruses, and diffusion constants for four viruses. 



Two cases are of some interest. The first is turnip yellow 

 mosaic virus. This has been studied by Markham and Smith 

 (1949). Preparations of this virus can be separated into two 

 layers in the ultracentrifuge. The upper layer contains no nucleic 

 acid and is not infectious, whereas the lower, denser layer is 

 nucleoprotein, which is infectious. The serological properties of 

 the two layers are the same, and the diffusion constant of the 

 upper layer is 1.51 X 10~^ cm^/sec, of the lower 1.55 X 10~^ So 

 the particle size is slightly different. Yet the sedimentation con- 

 stant for the lower layer is 106 S, and for the u])i)er layer it is 

 50 S. There is, therefore, a considerable difference in virus mass. 

 This can, in part, be related to the nucleic acid. X-ray studies 

 described in the next section indicate that, in addition, there is a 

 higher density. 



A second case of interest is the Rothamsted variety of tobacco 

 necrosis virus for which Ogston (1942) finds two sedimentation 

 constants of 240 and 51 S. Bawden and Nixon (1951) report 

 that these are two spherical components of diameter 370 and 

 180 A. 



In comment on this work it should be pointed out that the 

 basic physical methods of size and shape measurement by motion 

 studies are vulnerable to errors of interpretation. It has already 

 been seen that hydration and axial ratio can be confused. It is 

 quite likely that a reconsideration of the actual physical factors 

 involved may change detailed points of the measvu-ement analy- 

 sis. Thus the recent theory of Kirkwood and Shumaker (1952) 



