DONNAN EQUILIBRIUM AND THE PHYSICAL PROPERTIES 



OF PROTEINS. 



I. Membrane Potentials. 



By JACQUES LOEB. 

 {From the Laboratories of The Rockefeller Institute for Medical Research.) 



(Received for publication, March 14, 1921.) 

 I. INTRODUCTION. 



The different physical properties of proteins, such as osmotic 

 pressure, swelling, and viscosity, vary in a similar way under the 

 influence of changes in the concentration of hydrogen ions or under 

 the influence of neutral salts or under the influence of the valency of 

 the ion in combination with the protein. This fact suggests that 

 the variations of these three physical properties may have a common 

 cause, so that if this cause were known for one of these properties it 

 would, perhaps, be known for all of them. The clues tentatively 

 suggested have thus far either proved wrong or inaccessible to quan- 

 titative tests. 



In the last paper^ the writer showed that a measurable potential 

 difference is found when we separate a protein solution from a watery 

 solution (free from protein) by a collodion membrane. When the 

 protein in solution is a protein-acid salt {e.g., gelatin chloride) the 

 protein solution is positive and the water is negative; when the pro- 

 tein exists in the form of a metal proteinate the protein solution is 

 negative and the watery solution positive. The turning point seems 

 to lie at the isoelectric point. Quantitative measurements of these 

 p.D. between gelatin chloride solutions and the outside watery solu- 

 tion with which the gelatin chloride solution was in osmotic equilib- 

 rium revealed the fact that these potential differences varied in a 

 similar way as the osmotic pressure, the swelling, or the viscosity 



1 Loeb, J., /. Gen. Physiol., 1920-21, iii, 557. 



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