PROTEINS 103 



acid with magnesium or sodium amalgam) is added to a pro- 

 tein solution, and concentrated sulphuric acid added so as to 

 form a layer at the bottom of the test tube, a violet ring will 

 form at the juncture of the two liquids. This test is due to the 

 tryptophane in the protein molecule, and only those proteins 

 containing this amino acid will respond to the test. When 

 glacial acetic acid is used, the test is believed to be due to the 

 presence as an impurity of glyoxylic acid, HCO COOH, or 

 other aldehydes. Nitrates, nitrites, chlorides, chlorates and some 

 other substances interfere with this test. 



Sulphur Test. If concentrated sodium hydroxide and lead 

 acetate are added to a protein solution and the mixture boiled, 

 a brown or black color appears. The unoxidized sulphur of 

 the cystein or cystin is split off and combines with the lead to 

 form the dark brown or black lead sulphide. 



Precipitation Reactions. Proteins may be precipitated by a 

 variety of reagents. The behavior of protein solutions with 

 precipitation reagents, and in fact many other properties of 

 protein solutions indicate that the proteins do not form true 

 solutions such as that of sodium chloride in water. They form 

 colloidal solutions. 



Heat. Many proteins are precipitated by heat. A slightly 

 acid reaction, and the presence of salts is desirable if precipi- 

 tation is to be complete. Alkaline solutions of proteins do not 

 precipitate on boiling. Neutral solutions, especially if salts 

 have been removed by dialysis, will precipitate only imperfect- 

 ly. Solutions of some proteins, i.e., casein, gelatine and sec- 

 ondary derived proteins such as proteoses and peptones do not 

 precipitate on boiling. In general for each protein there is a 

 specific precipitation temperature, but experimental conditions 

 such as the amount of salts present, the rapidity of heating and 

 other factors cause rather wide variations in the precipitation 

 temperature. A protein precipitated by boiling in weak acid 

 solution cannot readily be redissolved. Some as yet unknown 

 change has taken place in the protein molecule, and the sub- 



