144 



PROTEINS 



The theory for the precipitation of proteins is based on the amphoteric 

 character of the proteins, i.e., the presence of both basic and acidic groups 

 in the molecule. If the protein is more basic than the reagent, the precipi- 

 tate is a protein salt of that reagent. If it is more acidic than the reagent, 

 the precipitate comes down as proteinate; that is, the protein behaves as 

 an acid and the precipitant acts as a base. For each protein there is 

 a pH value called the iso-electric point, on one side of which the protein 

 acts as a base and on the other side as an acid; for example, the iso- 

 electric point of gelatin is at the pH value of 4.7. Below 4.7, gelatin 

 acts as a base and is precipitated by acids as a gelatin salt such as gelatin 

 hydrochloride. Above 4.7, gelatin behaves as an acid and is precipitated 

 as a gelatinate such as sodium gelatinate. The two types of precipita- 

 tion may be represented in simplified form by the following equations. 



By acids : RNH. + HCl -^ RNHo-HCl 

 By bases : R'COOH + NaOH -^ R'COONa 



In place of hydrochloric acid in the acid precipitation, we may have 

 acids such as picric, tannic, tungstic, phosphotungstic, etc., which form 

 more insoluble compounds with proteins. Instead of sodium hydroxide, 

 calcium or barium hydroxide may be used to give the corresponding 

 calcium or barium salt. The metallic salt of the protein may also be 

 formed by adding a soluble salt of the metal to the sodium hydroxide- 

 protein solution. For example, if lead acetate, one of the best protein 

 precipitants, is added to a solution, lead proteinate is formed and, since 

 this is insoluble, a precipitate is produced. 



The above theory of precipitation and salt formation assumes the 

 formation of an excess of positive charges on the protein molecule 



RNHo + H2O -> RNH3+ + OH- 



below the iso-electric point and an excess of negative charges on the 

 protein molecule 



R'COOH -^ R'COO- + H+ 



above the iso-electric point. Under the first condition, the protein reacts 

 with negative ions, e.g., Cl~, to form a protein salt; under the second, 

 it reacts with positive ions, e.g., Na + , to form a proteinate. At the 

 iso-electric point the positive charges equal the negative charges; hence 

 the protein combines with neither acid nor base. Above or below the 

 iso-electric point the protein is unbalanced and therefore combines with 

 oppositely charged ions. 



Denaturation 



Neurath and associates define protein denaturation as "any nonpro- 

 teolytic modification of the unique structure of a native protein, giving 



