196 



SOLUTION AND PRECIPITATION OF PROTEINS 



The solution was kept at 20° for 16 hours and then allowed to settle, 

 for 24 hours at 20° in 100 cc. graduate cylinders. The dry weight of 

 the sediment was determined and this weight when deducted from 

 the dry weight of 1 gm. isoelectric casein, namely, 0.870 gm., was the 

 amount that had gone into solution after a correction was made for 

 the free NaCl held in 2 cc. solution which was arbitrarily assumed not 

 to have been removed. Though this latter correction was somewhat 

 arbitrary, it could have caused a noticeable error only when the con- 

 centration of the salt solution exceeded m/64. For the solutions of 

 m/64 and below this error was negligible. Table II gives the number 

 of milligrams of casein which had gone into solution. 



TABLE II. 



The main fact is that a slight increase in the concentration of NaCl 

 causes already a noticeable drop in the rate of solution. Thus m/1024 

 NaCl causes already a noticeable diminution in the solubility of a 

 1 per cent solution of casein chloride of pH 2.12 at 24°C. 



These observations then indicate that the solution of solid particles 

 of casein chloride is brought about by the ultimate elements being 

 forced apart mechanically through the process of swelling. The 

 force acting in this swelling is the hydrostatic pressure of the water 

 which is forced into the interstices of the solid particles by the os- 

 motic pressure in the interstices between the casein ions. Procter 

 and Wilson have shown that the application of Donnan's theory of 

 membrane equilibrium accounts quanitatively for this swelling on 

 the assumption that swelling is caused by the excess of the osmotic 

 pressure inside the gel over that of the surrounding solution. As 

 soon as the osmotic pressure in the gel exceeds the forces of cohesion 

 between the casein ions of the gel the casein ions constituting the 

 gel are separated. 



The question then arises: How can the Donnan effect stabilize the 

 particles of casein chloride in solution, and how can we explain the 



