VISCOMETRY 



Table III 

 The Shapes of Protein Molecules 



hydrated in order to obtain the shapes from both intrinsic viscosity 

 data and friction ratios. Since both friction ratios and intrinsic vis- 

 cosities are increased by hydration, both sets of calculations could be 

 in error. Mehl et al. tried to assess the effect of hydration. Bull and 

 Cooper (5) discussed the issue in greater detail, and presented a 

 method of estimating the average degree of hydration for proteins from 

 a consideration of essentially the same data. Hov/ever, since their 

 method involved an assumption not consistent with the Simha and 

 Perrin equations, for the validity of both of which direct experimental 

 evidence has been presented (16), their treatment serves more to 

 emphasize the need for a reliable method of determining hydration than 

 to provide such a method. One of the serious obstacles to the under- 

 standing of the physical properties of proteins and other biological 

 materials remains the absence of adequate means for assessing quanti- 

 tatively the role of hydration. 



The question of the validity of Simha's equation was recon- 

 sidered in an investigation of the physical properties of tobacco mosaic 

 virus preparations (16). Two preparations of the virus in different 

 states of aggregation were studied in the viscometer and also in the 

 electron microscope. The one preparation gave a relatively low in- 

 trinsic viscosity and the electron micrograph showed rod-shaped 

 particles with a unimodal distribution of lengths. The other prepara- 



