SIZE, SHAPE, AND HYDRATION OF VIRUSES 57 



of 151 A was found for all, except the cucumber viruses, which 

 had a somewhat smaller width of 146 A. 



Bernal and Carlisle (1948) have made most interesting studies 

 on turnip yellow mosaic virus. This virus is remarkable in that, 

 as has been previously mentioned, the crystalline preparations 

 separate ultracentrifugally into two factions, one contains nucleic 

 acid and is infectious, and one has no nuclei acid and is non- 

 infectious but has the virus serological properties. Bernal and 

 Carlisle found that both classes of crystalline preparations had 

 the same kind of X-ray structure, but that the virus repeat 

 dimension for the infectious ty])e was "^^S A, whereas for the non- 

 infectious, nucleic-acid-free particles it was 238 A, or 10 A 

 bigger. This remarkable fact suggests to Bernal and Carlisle that 

 the nucleic acid holds the protein in a more tightly bound con- 

 figuration. On drying, the interparticle distance shrinks and is 

 the figure quoted above. When wet, the distance increases by 

 77 A or so. 



Very interesting interparticle regularities were also observed 

 for TMV and will be discussed later. 



In the case of tobacco mosaic virus, Bernal and Fankuchen 

 were able to secure sufficiently accurate orientation to observe 

 reflections which correspond to some structure inside the unit 

 cell. The difficulties of analyzing such reflections are very 

 great, but they propose that in the hexagonal, unit cell there 

 are platelets of dimensions 44 X 44 X 22 A which are arranged 

 in regular order throughout the cell. The possibility of this type 

 of analysis of internal structure makes further X-ray dift'raction 

 work on viruses of great importance. 



These X-ray studies can also be used to measure hydration, 

 or at any event to act as a check on proposed values of hydra- 

 tion. If observations on spherical viruses, or oriented, long, thin 

 viruses, are made, the interparticle distance is found to be 

 a function of concentration. In the case of TMV it was found 

 that, as the virus was suspended in higher and higher concentra- 

 tions of ammonium sulphate, the interparticle distance fell to a 

 value of 178 A. In the dry gel preparations, in which the virus 

 dries in an oriented way, the separation was found to be 151 A, 



