Donnan Equilibrium and the Physical Properties of Proteins

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 ~th 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 aH of them. The clues tentatively suggested have thus far either proved wrong or inaccessible to quantitative tests. In the last paper 1 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 protein 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 solution with which the gelatin chloride solution was in osmotic equilibrium revealed the fact that these potential differences varied in a similar way as the osmotic pressure, the swelling, or the viscosity