STUDIES IN THE PHYSICAL CHEMISTRY OF THE 



PROTEINS. 



I. The Solubility of Certain Proteins at Their Isoelectric 



Points. 



By EDWIN JOSEPH COHN.* 



{From the Laboratory of Physical Chemistry, Harvard Medical School, Boston.) 



(Received for publication, May 23, 1922.) 



In 1899 W.B.Hardy observed that "under the influence of a constant 

 current the particles of proteid in a boiled solution of egg white move 

 with the negative stream if the reaction of the fluid is alkaline; with 

 the positive stream if the reaction is acid" (1). The protein in the 

 "boiled solution of egg white" had, of course, been denatured, but 

 Pauli (2, 3) later investigated the direction of migration of unde- 

 natured serum albumin in an electric field, and found that it, too, moved 

 toward the cathode in an alkaline solution and toward the anode in an 

 acid solution. Presumably the charge on the protein was negative 

 in alkaline, but positive in acid solution. 



Michaelis was the first investigator to determine the hydrogen ion 

 concentration at which the migration of a protein changed its direction. 

 He studied serum albumin, and found that the change was abrupt 

 (4). It occurred within exceedingly narrow limits. At a hydrogen 

 ion concentration of 2.1 X 10~^ serum albumin migrated toward the 

 cathode. At smaller hydrogen ion concentrations than 1.9 X 10~^ 

 it migrated toward the anode. At 2.0 X 10~^ the protein appeared 

 to be in an isoelectric condition (5) and this hydrogen ion concentra- 

 tion was accordingly termed the isoelectric point. 



The movement in an electric field of particles charged with respect 

 to their surrounding medium is termed cataphoresis. From catapho- 

 resis the sign, and the average magnitude, of the charge can be deter- 

 mined, but not its nature or origin. The nature of the charge of the 

 protein molecule will be discussed in a later section of this paper from 

 a theoretical point of view. 



* Fellow in Chemistry of the National Research Council. 



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