COLOUR TESTS FOR PROTEINS 85 



produce simpler bodies. These are the peptones, very 

 soluble substances, which cannot be salted out of their 

 solutions, but are precipitated by phosphotungstic, phos- 

 phomolybdic, or tannic acids. 



(d) Poly pep tides are a group of products of protein 

 cleavage beyond the peptone stage. The majority are 

 synthetical substances, which cannot here be dealt with. 



3. Colour Tests for Proteins. (i) The biuret reaction. 

 A solution of a protein, when made strongly alka- 

 line with caustic potash, gives a red, pink, or violet 

 colour when a few drops of copper sulphate is added. 

 With many albumins the colour is blue or violet, with 

 proteoses and peptones red or pink. 



(2) Millon's reagent (a solution of mercurous nitrate 

 in nitric acid) gives a pink or dark-red colour when 

 boiled with a protein which contains the oxyphenyl or 

 tyrosine group. The latter is absent from gelatine and 

 certain proteoses and peptones. 



(3) The Xantho-proteic reaction. Strong nitric acid, 

 added to a solution of protein and heated, gives a yellow 

 solution or a yellow precipitate. If ammonia or caustic 

 soda is added afterwards, a deep orange or reddish brown 

 colour is produced. 



(4) Proteins containing sulphur, when boiled with a 

 solution of caustic soda containing lead acetate, gives a 

 black or brownish precipitate, or the solution becomes 

 dark-coloured owing to the formation of lead sulphide. 



(5) Reaction of Molisch. When a few drops of an 

 alcoholic solution of a-napthol is added to the solution 

 of a gluco-protein, and strong sulphuric acid added to 

 the mixture, a violet colour is produced. The sulphuric 

 acid splits off a carbohydrate from the gluco-protein, 

 and converts it into furfurol. The latter then gives a 



