Arda Alden Green 



219 



determined, but are corrected for slight amounts of both negatively and 

 positively charged molecules present with the neutral molecule even at the 

 pH of minimum solubility. The solubility of casein at various pH values 

 may also be described by equation 2. This equation has been applied success- 

 fully to the data of Linderstr0m-Lang and Kodama* and of S0rensen and 



Sladek.^" 



Other Methods of Precipitation 



Temperature, pH, isoelectric point, and concentration of solvent must also 

 be considered when other methods of separation or precipitation of proteins 



40 



_L 



oEQCi ALBUniN 8.7 55 

 • HEMOGLOBIN 1220 1335 



_L 



A, 



5.75 

 2.77 



4.5 5.0 



pH OF EGG ALBUMIN SOLUTIONS 



5.5 



BO 



5.5 7.0 



pH OF HEMOGLOBIN 50LUTI0N5 



7.5 



Fig. 3. The solubility of hemoglobin and of egg albumin in concentrated 

 salt solutions of varying pH. 



(From Green, A. A.: Jl. Biol. Chem. 93:524, 1931.) 



are employed. These other methods will not be discussed here in any detail. 

 They include colloidal adsorption; differential denaturation, especially by 

 heat; combination with specific substances; or precipitation by such organic 

 solvents as acetone and methyl or ethyl alcohol. Alcohol at low temperatures 

 has been used most recently by Cohn and his co-workers" for the separation 

 of plasma proteins. Antibodies may be specifically precipitated by combi- 

 nation with the antigen. Heidelberger and KendalP have prepared pure 

 pneumococcus antibody by precipitation with the type-specific carbohydrate 

 and subsequent solution in salt solutions. Northrop" has recently crystallized 

 diphtheria antitoxin by digesting the toxin from the toxin-antitoxin complex 

 with the aid of trypsin and subsequently precipitating fractionally with 

 ammonium sulfate. All of the above-mentioned methods may be used in 

 combination with or alternately with isoelectric or salt precipitation. 



