PROTEINS. I I 5 



it in a little water. To remove the ammonium sulphate, which 

 adhered to the precipitate and is now in solution, add barium car- 

 bonate, boil, and filter off the precipitate of barium sulphate. Con- 

 centrate the proteose solution to a small volume 1 and make the 

 following tests : 



1 i ) Biuret Test. 



(2) Precipitation by Nitric Acid. What would a precipitate at 

 this point indicate? 



(3) Precipitation by Trichloracetic Acid. This precipitate dis- 

 solves on heating and returns on cooling. 



(4) Precipitation by Picric Acid. This precipitate also disap- 

 pears on heating and returns on cooling. 



(5) Precipitation by Potassio-mercuric Iodide and Hydrochloric 

 Acid. 



(6) Coagulation Test. Boil a little in a test-tube. Does it 

 coagulate ? 



(7) Acetic Acid and Potassium Ferrocyanide Test. 



The solution containing the peptones should be cooled and fil- 

 tered, and the ammonium sulphate in solution removed by boiling 

 with barium carbonate as described above. After filtering off 

 the barium sulphate precipitate, concentrate the peptone filtrate to 

 a small volume 1 and repeat the test as given under the proteose solu- 

 tion, above. In the biuret test the solution should be made very 

 strongly alkaline with solid potassium hydroxide. 



PEPTIDES. 



The peptides are " definitely characterized combinations of two 

 or more amino acids, the carboxyl (COOH) group of one being 

 united with the amino (NH 2 ) group of the other with the elimina- 

 tion of a molecule of water." These peptides are more fully dis- 

 cussed on pages 66 and 113. 



REVIEW OF PROTEINS. 



In order to facilitate the student's review of the proteins, the 

 preparation of a chart similar to the model on p. 117 is recom- 

 mended. The signs + and -- may be conveniently used to indi- 

 cate positive and negative reactions. 



x lf the proteoses are desired in powder form, this concentrated proteose solu- 

 tion may now be precipitated by alcohol, and this precipitate, after being washed 

 with absolute alcohol and with ether, may be dried and powdered. 



