PROTEINS 271 



inside of the test tube so as to make a layer beneath the aqueous 

 solution. A reddish violet or purple ring will develop at the 

 juncture of the two liquids. Shake the tube gently so as to mix 

 the two layers. The color will spread throughout the entire 

 solution. 



This test is often performed by adding glacial acetic acid in 

 place of a solution of glyoxylic acid, as glyoxylic acid is usually 

 present in glacial acetic acid as an impurity. It is more satis- 

 factory, however, to use a solution of glyoxylic acid, which may 

 be prepared by reducing oxalic acid with sodium amalgam or 

 magnesium. The reaction is due to tryptophane, which is present 

 in most proteins. 



5. Molisch Test. The student is already familiar with the 

 Molisch test from his work on carbohydrates. It is given by 

 many proteins and is supposed to indicate the presence of a car- 

 bohydrate group in the protein molecule. To about 3-4 c.c. of 

 albumin solution add 2-3 drops of 15% alcoholic oc naphthol. 

 Incline the tube and pour down the side a few cubic centimeters 

 of concentrated sulphuric acid to form a layer at the bottom 

 of the tube. Note the result. 



Precipitation Tests. 



6. By Heat. Heat about 3 c.c. of albumin solution to boiling. 

 If no precipitation occurs, add 1 drop of 1% acetic acid and boil 

 again, repeating the process until coagulation occurs. Repeat 

 the experiment, adding about 1 c.c. saturated sodium chloride 

 solution to the albumin solution. The presence of salts is fa- 

 vorable for coagulation. 



In removing the protein from a solution the acid is added after 

 heating, as otherwise acid metaprotein, which does not coagulate 

 on boiling, may be formed. Proteoses, peptones, casein and a few 

 other proteins are not coagulated by heat. Heat another portion 

 of albumin solution, first adding 1-2 drops of concentrated 

 sodium hydrate. 



7. Concentrated Mineral Acids. Prepare three test tubes each 

 containing a few cubic centimeters of albumin solution. Add 



