190 SOLUTION AND PRECIPITATION OF PROTEINS 



In order to gain more definite information concerning the nature of 

 the forces involved in the two cases it seemed advisable to supplement 

 these investigations by a study of the mechanism of the solution of 

 proteins. We have studied the mechanism of solution of granules of 

 isoelectric casein in acid and in alkah and find that this mechanism 

 is entirely different in the two cases. The solution of casein chloride 

 is controlled by forces connected with the Donnan equilibrium; and 

 it also happens that casein chloride when in solution can be precipi- 

 tated by low concentrations of electrolytes; e.g., m/8 NaCl, On 

 the other hand, the solution of sodium caseinate is apparently con- 

 trolled by the forces of chemical attraction between water and certain 

 groups of the casein molecule; and it happens that very high con- 

 centrations of electrolytes, e.g., 2\ M NaCl or Z\ m LiCl, may not 

 be sufficient for precipitation. • 



II. The Precipitation and Solution of Casein Chloride. 



1 per cent solutions of casein chloride of pH 2.2 were prepared in 

 different concentrations of salts in water of about the same pH. That 

 concentration was determined which causes an almost instantaneous 

 complete precipitation of the protein from the originally milky 

 solution so that the supernatant liquid became as clear as water. 

 These concentrations were as follows : 



NaCl about m/8 



NaNOj about m/8 



CaClj about m/8 



Na trichloracetate about m/16 



Naj SO4 about m/32 



Though the results are only semiquantitative, the validity of 

 Hardy's rule and the valency effect are easily recognizable. Beside 

 these two effects, some constitutional effects of the anion (such as the 

 trichloracetate) may exist. It is also obvious that the concentrations 

 of electrolytes required for instantaneous, complete precipitation 

 of casein chloride are considerably lower than those required for the 

 precipitation of Na caseinate from their watery solution, so that we 

 can be sure that in the case of casein chloride we are dealing with 

 a representative of the "suspension" group of colloids, in the sense 

 of Noyes. 



