700 PHYSICAL CHEMISTRY OF THE PROTEINS. I 



(though possibly not the flocculation) and therefore the apparent 

 isoelectric point of tuberin and serum globulin to a slight degree. 

 The reason for this appears from a theoretical consideration of the 

 nature of the charge on the protein molecule.^ 



As a result of the unsatisfactory nature of the experimental methods 

 that have been employed in determining the isoelectric point of the 

 slightly soluble proteins, and in view of the manifest importance and 

 significance of the isoelectric point for the interpretation of other 

 aspects of the physical chemistry of the proteins, it seems necessary 

 to base these studies upon better criteria of the identity of the proteins 

 under investigation. 



Theoretical. 



Hardy had been led to examine the cataphoresis of denatured al- 

 bumin from a consideration of the contemporary conceptions of 

 colloidal chemistry. Picton and Linder (15) had "established 

 . . . . that the direction of the movement of colloidal particles 

 under the influence of an electric current is determined by their chem- 

 ical nature," and Hardy concluded^ that "proteid molecules seem 

 therefore to act as basic or acid particles according to the circumstances 

 in which they find themselves. " 



In the same year in which Hardy published these conclusions 

 Bredig (16) extended the theory of electrolytic dissociation to the 

 case of molecules that act both as acids and as bases; that is, to am- 

 photeric electrolytes. The amphoteric nature of the amino-acids 

 and also of the proteins had already been recognized (17, 18, 19). 



^ If the assumption be made that the charge on the protein is due to its dissocia- 

 tion as an amphoteric electrolyte, then the cation of any salt will depress the dis- 

 sociation of any protein through its common ion on the alkaline side of the iso- 

 electric point, and the anion on the acid side. Sorensen (13) has used this phe- 

 nomenon as a method of determining the isoelectric point of egg albumin; the 

 isoelectric point coinciding with the hydrogen ion concentration that is unchanged 

 by the addition of a neutral salt. I have attempted to apply the method to 

 tuberin (14) but it is not entirely suitable to the globulins. Moreover, even where 

 the method is applicable for the determination of the isoelectric point in this way, 

 the effect of a common ion in depressing dissociation and thereby affecting solu- 

 biUty need not be identical on both sides of the isoelectric point. 



2Hardy (l),p. 297. 



