Osmotic Pressure Measurements on Proteins in Urea Solutions.

In the development of the theory of solutions, it has often happened that the use of other solvents than water has led to results that are entirely obscured in aqueous solutions. 1 It seems desirable on this account to study the physical chemical properties of proteins in such other solvents as are capable of dissolving them in appreciable quantities. A few such pure and mixed solvents that might be employed are anhydrous formic acid, liquid phenol, alcohol-water mixtures and urea-water mixtures.

As a start on this general problem, measurements of the osmotic pressure of proteins in urea solutions were undertaken. The 2 immediate objectives were to determine if changes in the state of aggregation of proteins could be detected, and, to develop a method for the determination of the molecular weights of proteins that are not soluble in water in the absence of electrolytes. Up to the present, the only means of fairly accurately determining the molecular weights of proteins (colloids in general) that have been developed are by osmotic pressure 2 determinations and by the ultracentrifuge 3 method of Svedberg. In both of these methods pure isoelectric solutions of proteins are required to obtain correct results. Only relatively few proteins are soluble in water in the isoelectric condition. Urea solutions have a very powerful solvent action on most proteins. If the protein is a pure, electrolyte free sample, the resulting solutions show only a very slight conductivity indicating that the dissolved protein is unionized. This makes it possible to make the osmotic pressure measurements with the elimination of the Donnan membrane equilibrium as an interfering factor.