COLOUR REACTIONS 297 



acetate of copper, the chloride of mercury and the acetates of 

 lead and zinc. 



2. Colour reactions. 



These reactions depend on the fact that certain groups or 

 radicles in the protein molecule produce characteristic colours 

 with suitable reagents. The reactions may also be employed 

 for detecting these same groups in the decomposition products 

 of the proteins, with the object of determining how far the 

 decomposition has gone, and whether it has been sufficiently 

 deep-seated to destroy this grouping or not. The following is 

 a list of the more important colour reactions : — 



(i) Biuret Reaction. — This is the bluish-violet colour pro- 

 duced by adding copper sulphate to an alkaline solution of a 

 protein. Unchanged proteins give a bluish-red, whilst altered 

 proteins, such as the peptones, give a pink. 



The colour is given by the substance biuret itself, whose 

 composition is expressed by the formula NHoCO.NH . CONH2, 

 and by similarly constituted compounds containing two 

 — CO . NH — groups connected together through a carbon, 

 nitrogen, or sulphur atom. 



(ii) Millons Reaction. — A solution of mercuric nitrate con- 

 taining nitrous acid added to a solution of a protein pro- 

 duces a precipitate which turns pink or red. This reaction 

 is connected with the phenolic group of the tyrosine com- 

 plex in the protein molecule ; it may also be used as a test 

 for tyrosine. 



(iii) Xanthoproteic Reaction. — Protein solutions treated with 

 concentrated nitric acid develop a yellow colour which is in- 

 tensified by heating, and is changed to orange by ammonia. 

 This reaction is likewise connected with the tyrosine complex. 



(iv) Adanikiewiczs Reaction. — The addition of concentrated 

 sulphuric acid to a solution of a protein dissolved in acetic acid 

 produces a reddish-green or violet colour. This reaction is 

 characteristic of the tryptophane group, and is produced by 

 the interaction of this with glyoxylic acid contained in the 

 acetic acid ; the reaction may be intensified by replacing acetic 

 by glyoxylic acid in the test. 



(v) Liebermann's Reaction. — Proteins, which have been 

 previously extracted with alcohol and ether to remove fats, 

 on warming with concentrated hydrochloric acid, develop a 



