EDWIN JOSEPH COHN 703 



At the hydrogen ion concentration that corresponds to the isoelectric 

 point a definite ratio obtains between the total amount of the acid 

 dissociation and the total amount of the basic dissociation. This 

 ratio obtains irrespective of how many acid valences of different 

 strength or how many basic valences of different strength are involved. 

 For at any one hydrogen ion concentration the number of active acid 

 constants may be considered equal to one constant, Ka, and the active 

 basic constants to another constant, Kb (31). For this reason the 

 multivalent protein may be treated as a simple ampholyte at its iso- 

 electric point. We shall so consider it in this paper, since we are, for 

 the moment, only concerned with the solubility of certain proteins 

 at their isoelectric points. 



Michaelis pointed out that the precipitation of a protein was at 

 a maximum at its isoelectric point, since dissociation was at a minimum 

 (25). This follows from equation (5) if we define maximum precipita- 

 tion as minimum solubility, and assume that protein is as a rule more 

 soluble in the dissociated than in the undissociated state. According 

 to this conception a protein should be more soluble the greater its 

 amphoteric constants. I have shown (31) that this was the case 

 for different classes of proteins in a previous communication, by using 

 the acid- and the base-combining capacity of a protein as a measure 

 of its amphoteric strength.* 



In order to pursue this investigation further it was necessary to 

 correlate the solubility of proteins with their dissociation. This 

 involved accurately determining the solubility of a number of proteins 

 at their isoelectric points, and quantitatively distinguishing between 

 the concentrations of dissociated and undissociated protein. Let 

 the solubility of a protein, P, be S. This solubility is made up of the 

 concentration of the undissociated protein molecule and of the dis- 

 sociated protein ions, in the saturated solution. We may write 



S = (HPOH) + (HP+) + (POII-) (9) 



*In practise this was estimated by titrating electrometrically 1 gm. of protein 

 'with NaOH and HCl and measuring the rate of change of the hydrogen ion con- 

 centration (by the slope of the tangent to the titration curve) at the isoelectric 

 point. These estimates are therefore subject to revision when sufficiently accurate 

 data of the molecular weights of the proteins are available. 



