240 Biological Chemistry. 



benzene nucleus in the protein molecule. With gelatin, the 

 xanthoproteic acid is either absent or very faint. This 

 protein yields very small amounts of aromatic derivatives 

 on hydrolysis. 



(2) " Millon's reagent " gives with proteins a white 

 precipitate which turns red on boiling, and produces a 

 coloration in the supernatant fluid. This reaction is also 

 said to be due to the presence of an aromatic group 

 in the molecule. The reagent is prepared by dissolving 

 mercury in an equal weight of nitric acid (sp. gr. 1 ! 4) by 

 the aid of gentle heat, and then diluting the solution with 

 twice the volume of cold water. The liquid thus obtained 

 contains mercurous nitrate and nitrous acid. 



(3) An important reaction is that produced when a 

 few drops of very dilute copper sulphate solution are 

 added to protein solutions. A precipitate is produced 

 which dissolves on the addition of excess of potassium or 

 sodium hydroxide. If the mixture is then warmed, a fine 

 violet coloration is generally produced. Certain deriv- 

 atives of proteins, such as the proteoses, produce under 

 similar circumstances a rose-red colour. This reaction is 

 known as the " biuret reaction." It is probably con- 

 nected with the presence of peptide linkages in the mole- 

 cule, and is also yielded by substances other than proteins. 



(4) Certain colour reactions are ascribed to the pre- 

 sence of tryptophan in the molecule, and are only yielded 

 by such proteins from which this substance can be ob- 

 tained by hydrolysis with ferments. The chief tryptophan 

 reaction is due to Adankiewicz, and called by his name. 

 It was originally carried out in the following manner : 

 The solution of a protein (which yields tryptophan on 

 hydrolysis) is treated with an equal volume of concentrated 

 sulphuric acid, and twice the volume of glacial acetic acid ; 

 a reddish-violet colour is thereby produced. It was sub- 



