704 PHYSICAL CHEMISTRY OF THE PROTEINS. I 



provided the protein is quite uncombined with acid or with base. 

 If a part of the protein is combined with a base B, however, or a part 

 with an acid A, we must write^ 



S = (HPOH) + (HP+) + (POH-) + (BP) + (PA) (10) 



But let us first consider the case of an uncombined protein at its 

 isoelectric point. Its degree of dissociation is given by equation (5) 



VKa'Kb 

 This ratio is, as we have shown above, a con- 

 Kw 



stant for any protein at its isoelectric point. Walker showed that the 



. r • .• f • 'A -A K (HP+) (POH-) 



degree of dissociation of an ammo-acid as acid or as base, — ;; » 



" (HPOH) 



was a constant, independent of dilution from conductivity data. 



In the case of a protein that is relatively insoluble at its isoelectric 

 point the undissociated protein HPOH may be assumed to have a 

 definite solubihty. Moreover, this molecule dissociates, as we have 

 seen, to form a protein cation HP+, and a protein anion POH". 

 The solubility determined experimentally must be the sum of the 

 concentrations of the undissociated protein molecule and of the protein 

 ions. But if the concentration of the undissociated molecule is con- 

 stant, the degree of dissociation must also be constant at the isoelec- 

 tric point. Solubility must therefore be constant so long as the solu- 

 tion is saturated with respect to undissociated protein. 



Experiments now to be described have shown that this is the case 

 when the protein is uncombined with base or acid. The solubility of 

 casein and of the two globulins, tuberin and serum globulin, that have 

 thus far been investigated, was found to be constant at the respective 

 isoelectric points of the proteins, when the amount of protein precipi- 

 tate with which the solution was in heterogeneous equilibrium was 

 varied within w^ide limits.''' 



Only at the isoelectric point was solubility independent of the 

 amount of the protein in the system. At greater hydrogen ion concen- 



^ I am indebted to Prof. S. P. L. Sorensen for first calling my attention to the 

 convenience of treating solubility as an unknown function of concentration. 



"^ If the amount of the protein precipitate was increased beyond a certain point, 

 a secondary effect slightly increasing solubility was detected. 



