PROPERTIES 439 



(ferric hydroxide), the sulphate and 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 dilute copper sulphate to an alkaline solution 

 of a protein. Unchanged proteins give a bluish-violet, 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 NHgCO.NH.CONHa, 

 and by similarly constituted compounds containing two 

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

 nitrogen, or sulphur atom. 



(ii) Millon's 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 complex 

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

 tyrosine. The reagent may be prepared by dissolving some 

 mercury in twice its weight of nitric acid (sp. gr. 1-42), the 

 operation being performed in a fume cupboard. When the 

 action has ceased, the solution is diluted with twice its volume 

 of water. 



(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) Tryptophane Reaction. — This consists in mixing the 

 suspected solution with a little glyoxyhc acid * and carefully 



* Made by adding magnesium powder to a saturated solution of oxalic 

 acid. 



