108 THE CHEMICAL COMPOSITION OF THE BODY 



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 twoamino 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 



I 

 NH 2 CO 



2 C=0 = NH + NH S 

 \ \ 



NH 2 CO 



NH 2 



Urea. Biuret. Ammonia 



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

 fn 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 

 facilitate the making of the ring tests. Heller's ring test is most commonly 

 used to determine the presence of protein in urine. 



