PHYSIOLOGICAL CHEMISTRY 303 



the egg-white solution. A white coagulura occurs, which on warming 

 changes to a brick-red curd. 



This reaction differs from the xanthoproteic only in depending uponc/ I 

 the presence of the benzene ring with an hydroxyl group attached to it r\fa* 

 in addition, or, in other words, the phenolic ffroup. _ 



(d) The Glyoxylic Acid test (Hopkins' modification of Adamkiewiecz's 



Reaction). 



EXPERIMENT X. 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 fluid, 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). 



This has already been given under carbohydrates (q.v). Proteins containing 

 a carbohydrate moiety yield this test. The purple colour should be very pro- 

 nounced before the test is deemed positive. The green colour obtained plays 

 no part in the reaction. The test is not very reliable. 



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



(A) Ammonium Sulphate. 



EXPERIMENT XL 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 Am 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 Am 2 S0 4 therefore precipitates 

 globulins ; full saturation precipitates albumins. 



(B) Magnesium Sulphate. 



EXPERIMENT XII. Fully saturate (i.e. add crystals) the solution of 

 egg-white with MgS0 4 . A precipitate of globulin results. Filter. 

 Prove by heat coagulation and by fully saturating with Am 2 S0 4 that 

 protein (albumin) is left in the filtrate. Magnesium sulphate in full 

 saturation precipitates globulins, but not albumins (see table, p. 312). 



(C) Sodium Chloi'ide, Ammonium Chloride. These salts resemble 

 magnesium sulphate in their " salting out " properties. 



