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



Another form (1) better suited for methods based on the measure- 

 ment of concentration gradients is : 



^ = (1 - V.Wixl - xl) ^''^ 



4. Concentration Correction Factor 



The relation between the concentration of a particular component 

 in its respective plateau region at any time t and the original concen- 

 tration is, as shown by Svedberg and Rinde (20) : 



Ct = coixo/xty (16) 



where Xo and Xi are the respective radial distances of the meniscus 

 and the boundary at time t. 



D. EXPERIMENTAL REQUIREMENTS 



1. Preparation of Material 



Under usual conditions most macromolecules are electrically 

 charged with respect to the suspending medium and this circumstance 

 tends to decrease their sedimentation rate, by reason of the attrac- 

 tion and added drag of the more slowly sedimenting ions. The effect 

 can be very large, amounting to more than 20% in many cases. In 

 the equations given in Section C for determining particle weights and 

 sizes, it was assumed that no extraneous forces of this type were ac- 

 tive. However, it has been shown that the addition of sufficient 

 low molecular electrolyte is effective in repressing this Donnan effect. 

 In the case of proteins, the reduction in sedimentation rate by the 

 charge of the particle will amount to less than 1% if 0.2 mole of 

 sodium or potassium chloride is added for each 1% protein (85). 

 Also, it is known that pH, the concentration of the protein, tempera- 

 ture, high salt concentrations, and even the presence of another pro- 

 tein may influence the sedimentation constant and these factors must 

 be considered in any ultracentrifugal study. Another precaution 

 often neglected is that of "cleaning up" a preparation by filtration or 

 preferably by differential centrifugation before running it in the ul- 

 tracentrifuge. Aggregated or extraneous material of high sedimenta- 

 tion rate which will be precipitated before the boundary being studied 

 becomes differentiated can introduce error into the original deter- 



