196 PRACTICAL PHYSIOLOGY 



(6) Xanthoproteic Reaction. 



EXPERIMENT. To about 5 c.c. of the solution of egg-white add 

 a few drops of strong nitric acid ; a white precipitate results. 

 Warm this and the precipitate changes to a yellow curd. If little 

 protein is present no visible coagulation takes place. Cool under 

 the tap. Add a few drops of strong ammonia ; the yellow colour 

 changes to a brilliant orange. The name xanthoproteic (yellow 

 protein) will help the student to remember the colour obtained. 

 This test shows the presence of the benzene ring in the protein 

 molecule ; hence only proteins containing such a ring give this test. 



(c) Millon's Reaction. 



EXPERIMENT. Add a few drops of Millon's reagent (which 

 consists of a solution of mercurous and mercuric nitrates) to some 

 of the egg-white solution. A white coagulum forms, which on 

 warming changes to a brick-red curd. With some substances no 

 precipitation takes place but solution becomes red on heating. 



This reaction shows the presence of a benzene ring with an 

 hydroxyl group attached to it, i.e., of the phenolic group such as 

 occurs in tyrosine. 



(d) The Glyoxylic Acid Test (Hopkins' modification of Adamkie- 



wiecz's Reaction). 



EXPERIMENT. To some egg-white solution in a test tube add 

 about 1 c.c. of glyoxylic acid solution, and run in carefully without 

 mixing ordinary strong sulphuric acid. A violet ring is obtained at 

 the junction of the fluids, which extends into the supernatant egg- 

 white solution when the tube is gently agitated. 



This test depends upon the presence of tryptophan (indol amino- 

 propionic acid) in the protein molecule, and is only given by proteins 

 containing such a grouping. 



(e) The a-Naphthol Test (Molisch's test). (V, p. 210). 



Proteins containing a carbohydrate moiety yield this test. The purple 

 colour should be very pronounced before the test is deemed positive. The 

 green colour obtained plays no part in the reaction. The test is not very 

 reliable. 



VIII. Precipitation by Neutral Salts ("Salting out"). 



(A] Ammonium Sulphate. 



EXPERIMENT. To some egg-white solution add an equal amount 

 of saturated solution of ammonium sulphate = half saturation. A 

 white precipitate of globulin is produced. Filter ; keep the filtrate. 

 After washing the residue with saturated ammonium sulphate 

 dissolve it in a little water and boil. Note that the protein is 

 coagulated in fine flakes. Divide the filtrate : 



(a) Add crystals of (NH 4 ) 2 SO 4 in excess (full saturation). The 

 albumin is now salted out. 



(b) Boil ; flakes of coagulated protein show the presence of 

 coagulable protein (albumin). Half saturation with (NH 4 ) 2 SO 4 

 therefore precipitates globulins ; full saturation precipitates 

 albumins. 



