38 THE PROTEINS. [CH. II. 



20. Allow a few drops of serum to fall into about 10 cc. of 

 strong alcohol at room temperature. A white precipitate is formed. 

 Shake well and allow to stand for half an hour. Filter and treat 

 the precipitate with water. It does not dissolve. 



D. Colour Reactions. 



21. The Xanthoproteic reaction. To 3 cc. of the protein 

 solution in a test-tube add about one cc. of strong nitric acid. A 

 white precipitate is formed (see Ex. 40). Boil for a minute. The 

 precipitate turns yellow and partly dissolves to give a yellow solu- 

 tion. Cool under the tap and add strong ammonia or soda till the 

 reaction is alkaline. The yellow colour is turned to orange. 



NOTES. i. The essential features of the reaction are that a yellow colour 

 is obtained when the solution is boiled with strong nitric acid, and that this 

 yellow colour is intensified when the solution is made alkaline. 



2. The precipitate is due to the formation of metaprotein by the action 

 of nitric acid on albumins or globulins, this metaprotein being insoluble in 

 strong mineral acids. It follows that proteoses and peptones, etc., do not give 

 the precipitate with nitric acid. 



3. The yellow colour is due to the formation of a nitro-compound of some 

 aromatic substance, i.e. a substance containing the benzene ring. 



4. The aromatic substances in the protein molecule that are responsible 

 for the reaction are tyrosine, tryptophane and phenyl alanine. 



5. Oleic acid, olive oil and most vegetable oils give a well-marked xantho- 

 proteic reaction. 



6. To test for traces of proteins proceed as follows : Boil with nitric acid 

 and divide into two portions. Cool one portion and make it alkaline with 

 ammonia. Compare the colour of the two portions. The alkaline tube will 

 shew a faint yellow colour when only the merest trace of protein is present. 



22. Millon's reaction. Treat 5 cc. of the protein solution 

 with half its volume of Millon's reagent. A white precipkate is 

 formed. Cautiously heat the mixture. The precipitate turns 

 brick-red in colour, or disappears and leaves a red solution. 



NOTES. i. The essential feature of the reaction is the red colour on 

 heating. The white precipitate in the cold is due to the action of the mercuric 

 nitrate on the proteins. (See Ex. 9.) 



2. A white precipitate is also obtained with solutions of urea. (See 

 Exs. 341 and 342.) 



3. Sulphates give a white precipitate of mercurous sulphate. 



