94 E. G. P I C K E L S 



When it is desired to apply the equihbrium method to a particular 

 preparation, it should be first studied by the sedimentation velocity 

 method if possible to ascertain the degree of ultracentrifugal homo- 

 geneity. Unless the material is relatively monodisperse, it is not 

 ideally suited for study by the equilibrium method, although an 

 "average" value for the molecular weight can be obtained even with 

 mixtures (1). Columns of fluid are generally made shorter than for 

 velocity measurements in order to shorten the time required to attain 

 equilibrium, which is proportional to the square of the cell height 

 (radially) and inversely proportional to the diffusion constant (1, p. 

 56). For example, even with a fluid column only 5 mm. tall, almost 

 four days are required for a protein having a molecular weight of 

 40,000 and a diffusion constant of 8 X 10 ~^ 



E. INTERPRETATION OF RESULTS 



1. Significance of Frictional Ratio 



The frictional ratio f/fo can be obtained for a substance of known 

 molecular weight and sedimentation constant by comparing fric- 

 tional coefficients computed according to equations (6) and (8). 

 It is essentially the ratio between the actual frictional coefficient of a 

 substance in solution and the value it would have if the particles were 

 spherical and had an effective density in solution equal to the value 

 (o- = 1/V) used in the equations. The only density value generally 

 known is that for the dried, unsolvated material, and hence the de- 

 parture of the ratio from unity may be due to either or both solvation 

 and asymmetry of form. Values for some of the serum proteins are 

 given in Table I. From what is known about proteins, there is, in 

 accordance with equation (9), some assurance that hydration alone 

 would seldom account for an increase in the ratio by a factor of more 

 than about 1.2 or 1.25. By dividing the experimentally determined 

 ratio by the assumed ratio due to solvation, one obtains a new ratio 

 that may be used to gain some idea of particle shape through the 

 application of equations (10) and (11). For example, ratios of 1.10, 

 1.20, and 2 applied to equation (10) yield length to diameter ratios 

 of 2.9, 4.4, and 20, respectively. 



2. Determination of Homogeneity 



Ultracentrifugal homogeneity of a boundary can generally be 

 judged by comparing its shape and rate of spreading to values ob- 



