108 THE CHEMICAL COMPOSITION OF THE BODY 



merly thought that this reaction indicated the presence of a carbohydrate 

 group in the protein molecule, but this is now considered uncertain. 



e. Biuret Reaction. To 2 or 3 c.c. of egg-white solution in a test-tube an 

 equal volume of concentrated potassium hydroxide solution is added and 

 mixed thoroughly. Very dilute (2 per cent.) copper sulphate solution is 

 added until a purplish-violet or pinkish-violet color is produced. This 

 reaction is given by substances containing two amino groups in the molecule, 

 these groups being joined directly together or through a single atom of 

 nitrogen or carbon. Non-protein substances that contain the necessary 

 groups will of course respond to this test, which derives its name from the 

 fact that it is given by biuret, a substance formed on heating urea to 180. 



NH 2 



NH 2 CO 



/ / 



2 C=O NH + NH 3 



\ \ 



NH 2 CO 



I 

 NH 2 



Urea. Biuret. Ammonia. 



Proteins give this reaction since there are more than one CONH 2 group 

 in the protein molecule. Proteoses and peptones give a pink biuret re- 

 action, gelatin a rather blue reaction, and the ordinary proteins a purple. 



/. Molisch Reaction. This reaction is really a carbohydrate test, but is 

 given by some proteins and interpreted as indicating that such proteins 

 contain a carbohydrate moiety. The test is made as follows: 



Place about 5 c.c. of the solution to be tested in a test-tube, and add a 

 couple of drops of a 15 percent, alcoholic solution of a-naphthol. Incline 

 the tube and pour very carefully down the side about 5 c.c. of concentrated 

 sulphuric acid so that the two solutions are stratified. A blue or violet-red 

 ring is obtained in the area of contact of the solutions. 



4. Precipitation Reactions of the Proteins. a. Precipitation with 

 Concentrated Mineral Acids. Prepare four test-tubes which contain about 

 5 c.c. of egg-white solution. To these respectively add drop by drop con- 

 centrated sulphuric acid, hydrochloric acid, nitric acid, and acetic acid. 

 Note that the mineral acids precipitate the protein. The precipitation with 

 nitric acid is a frequently used protein reaction, and when carried out as 

 follows is known as Heller's ring test. The solution to be tested is placed 

 in a test-tube, the tube is inclined and about 5 c.c. of concentrated nitric 

 acid is poured carefully down the side of the tube so that the solution and 

 acid stratify. A white zone of precipitated protein is obtained between the 

 strata. An instrument known as the albumiscope has been devised to 



