PROTEINS 83 



groups is split off by the action of strong acids, the proteins will 

 no longer give a biuret reaction. Ammonium salts such as am- 

 monium sulphate interfere with the test, and should be decom- 

 posed by boiling the mixture with strong alkali before making 

 the test. Some of the proteins, e.g., the histones, and some of 

 the products of protein hydrolysis give a reddish color. 



Millonl&-TsL If a few drops of Millon's reagent (a solution 

 made by dissolving mercury in concentrated nitric acid) is 

 added to a protein solution, a yellowish preciptate forms. On 

 boiling, this precipitate turns rose pink. If the boiling is con- 

 tinued the pink color is destroyed and the precipitate turns 

 brown. At times the whole solution becomes pink. If the 

 protein happens to be insoluble, it will give the test quite as 

 well, turning a very decided pink or red. The test depends on 

 the presence in the protein of tyrosine, and is given only by 

 those proteins which contain this amino acid. Phenol or any 

 other compound which contains a hydroxyphenyl group will 

 give the test, however. The dihydroxybenzenes do not give the 

 test unless one hydroxyl group is substituted. Chlorides, alco- 

 hol or hydrogen peroxide will interfere with the test. Thus it 

 is not serviceable to test urine for protein, since urine contains 

 large amounts of sodium chloride. 



Xanthoproteic Test. If a protein is warmed with concen- 

 trated nitric acid the mixture turns lemon yellow. On the ad- 

 dition of alkali the color changes to a deep orange. The pro- 

 tein need not be in solution. Students of chemistry will recall 

 having performed this test upon themselves by getting con- 

 centrated nitric acid upon the fingers, producing the familiar 

 yellow spots. The skin is made of protein material. This test 

 depends upon protein constituents containing the benzene ring, 

 namely tyrosine, phejnylalanins. and tryptopj? *"*, The colored 

 substance is a nitro derivative of benzene. Any substance, pro- 

 tein or otherwise, containing a benzene ring will respond to 

 this test. 



Adamkiewicz or Hopkins-Cole Test. If glacial acetic acid, 

 or a solution of glyoxylic acid (prepared by reducing oxalic 



