NITROGENOUS COMPOUNDS 69 



because the necessary radicles do not enter into their compo- 

 sition. On the other hand, the colours are produced by some 

 non-protein substances which do contain these radicles. 

 The colour reactions are not, therefore, very characteristic of 

 the class. The more important are as follows : 



(1) Biiiret Reaction. A purple, violet, or, in some cases, a 

 red colour on adding a drop of cupric sulphate and excess of 

 caustic soda. This reaction depends on the formation of 

 biuret, NH(CO.NH 2 ) 2 . 



(2) Xanthoproteic Reaction. A yellow colour on heating 

 with concentrated nitric acid ; turns brown on addition of 

 ammonia. Applicable to solids and liquids. Depends on the 

 formation of nitrophenols. 



(3) Milloris Reaction. A pink colour, either in the cold or 

 after heating, on adding acid mercurous nitrate ; applicable to 

 solids and liquids ; positive results only with proteins which 

 contain a benzene nucleus. 



(4) VogeFs Reaction. Lead acetate gives a white precipitate 

 which turns black on boiling, owing to decomposition of the 

 lead protein salt with formation of lead sulphide. Negative 

 results with gelatine, peptones, and other proteins, which contain 

 no sulphur. 



(5) The Moliseh Reaction (see p. 15). This reaction is 

 characteristic of the carbohydrates, and only those conjugated 

 proteins which contain such radicles give positive results. 



(6) Adamkiewicz 1 (Hopkins and Cole's) Reaction. A red, 

 green, or violet ring on adding concentrated sulphuric acid to 

 a solution of the protein in glacial acetic acid. Positive 

 results only with proteins which contain tryptophane. 



(7) Lieberman's 1 Reaction. A blue coloration on warm- 

 ing with fuming hydrochloric acid. 



Dissociation. When the acid and alkali salts of the 

 proteins are " saponified " by the action of water (p. 68) the 

 proteins undergo hydrolysis to a greater or less extent, and 

 the properties of the products are not, therefore, exactly the 

 same as those of the original substances. These products 



1 For 6 and 7 the protein must be first purified from fat by treatment 

 with ether. 



