THE STABILITY OF PROTEIN SOLUTIONS 251 



NHj groups of the protein molecule are more active on the acid 

 side of the isoelectric point and the COOH or COO groups on the 

 alkaline side of the isoelectric point. The analogy with the soaps 

 would also suggest that the nature of the non-protein ion is of 

 importance for the solubility of a protein salt. This is found to 

 be true especially in the case of casein-acid salts, casein chloride 

 being more soluble than casein nitrate, and the latter more 

 soluble than casein trichloracetate. 



Until evidence to the contrary is furnished, we must consider 

 the possibility that the forces keeping proteins, such as gelatin 

 or crystalline egg albumin, in aqueous solutions are the same forces 

 which keep crystalloids in solution. The fact that gelatin 

 solutions set to a gel does not necessarily contradict this con- 

 clusion. When gelatin solutions approach the gel state, (i.e., 

 when they reach a high viscosity), the relative distance of the 

 protein molecules or ions from each other remains the same and 

 the affinity of the active groups of the protein ions or molecules 

 for water is not changed. The concentration of salt required 

 for precipitation remains also practically the same. 



3. At the isoelectric point the affinity of certain groups of the 

 gelatin molecule for water is a relative minimum, as is shown by 

 the fact that on standing at not too high a temperature, a 1 per 

 cent solution of isoelectric gelatin will become cloudy and the sus- 

 pended matter will settle; while this will not happen when the 

 pH is either above 4.8 or below 4.6. When a little alcohol is 

 added to such a solution near the isoelectric point, a rapid pre- 

 cipitation of the gelatin occurs. As soon as, however, the pH 

 is 4.4 or below, or 5.0 or above, the gelatin in solution remains 

 soluble even with an excess of alcohol, provided the anion of the 

 acid or the cation of the alkali added to the isoelectric gelatin is 

 monovalent (in the range of pH concerned), e.g., Cl, CH 2 COO, 

 H 2 PO 4 , HC 2 4 , etc., or Li, Na, K, NH 4 . When, however, these 

 ions are bivalent, e.g., SO 4 , Ca, Ba, the solubility of the gelatin 

 in alcohol is much less and the addition of a relatively small 

 amount of alcohol will cause precipitation of the gelatin. 1 The 

 addition of alcohol diminished the attraction between the watery 

 groups of the gelatin molecule or ion and the solvent, and where 

 these forces are small, as e.g. at the isoelectric point, or when the 



1 LOEB, J., J. Gen. PhysioL, vol. 3, p. 257, 1920-21. 



