626 PHYSIOLOGICAL CHEMISTRY. 



An alkaline solution of a protein will not coagulate on boiling ; a neutral 

 solution will do so partially ; a solution showing an acid reaction will be 

 coagulated completely on boiling, provided the quantity of neutral salt present 

 be not too small, and the protein solution not too dilute. 



Tests for simple proteins. 



a. Coagulation or precipitation tests. 



(Use a solution made by dissolving white of an egg in about 10 parts of a 

 2 per cent, sodium chloride solution.) 



1. Heat test. To 5 c.c. of protein solution add a few drops of 

 dilute acetic acid, and heat. The protein is completely coagulated. 



2. Heller's test. Place 1 c.c. of nitric acid in a test-tube, and allow 

 a few c.c. of protein solution to flow down the side of the tube, taking 

 care that the liquids do not mix. A white, opaque ring of coagu- 

 lated protein forms at the line of junction. (Strong sulphuric, 

 hydrochloric, and metaphosphoric acids coagulate proteins in the 

 same way.) 



3. To 5 c.c. of protein solution add solution of cupric sulphate ; 

 repeat with solutions of mercuric chloride, lead acetate, and silver 

 nitrate. In all cases coagulation takes place. (These reactions 

 explain the use of proteins, such as the white of egg, as antidotes in 

 cases of poisoning by metallic compounds.) 



4. To 5 c.c. of protein solution add a few drops of acetic acid and 

 some potassium ferrocyanide solution ; coagulation takes place. 



5. Saturate 10 c.c. of protein solution with ammonium sulphate ; 

 all proteins are precipitated except peptones. 



6. Solutions of picric, trichloracetic, phosphotungstic, phospho- 

 molybdic, tannic, taurocholic, and nucleic acids, potassium mercuric 

 iodide, alcohol, all precipitate proteins under special conditions. 



b. Color tests. 

 (Use any dry protein.) 



1. Xanthoproteic reaction. Heat a small quantity of protein with 

 concentrated nitric acid ; the protein, or the solution, turns yellow. 

 Allow it to cool, and add an excess of ammonia : the color changes 

 to orange. (Plate VIII., 1.) 



This reaction is due to the presence of the tyrosine and the trypto- 

 phane radicals in the protein molecule. 



2. Milton's reaction. Pour a few c.c. of water on a small quantity 

 of protein ; add 1 c.c. of Millon's reagent, and boil : a purple-red 

 color develops. (Millon's reagent is made by dissolving 10 grammes 



