PROTEINS 



(1) Precipitation by Picric Acid. To 5 c.c. of proteose-peptone solution in a 

 test-tube add picric acid until a permanent precipitate forms. The precipitate 

 disappears on heating and returns on cooling. 



(2) Precipitation by a Mineral Acid. Try the precipitation by nitric acid. 



(3) Coagulation Test. Heat a little proteose-peptone solution to boiling. 

 Does it coagulate like the other simple proteins studied? 



SEPARATION OF PROTEOSES AND PEPTONES 1 



Place 50 c.c. of proteose-peptone solution in an evaporating dish or casserole, 

 and half-saturate it with ammonium sulphate solution, which may be accom- 

 plished by adding an equal volume of saturated ammonium sulphate solution. 

 At this point note the appearance of a precipitate of the primary proteoses 

 (protoproteose and heteroproteose). Now heat the half-saturated solution and 

 its suspended precipitate to boiling and saturate the solution with solid am- 

 monium sulphate. At full saturation the secondary proteoses (deuteroproteoses) 

 are precipitated. The peptones remain in solution. ^ 



Proceed as follows with the precipitate of proteoses: Collect the sticky 

 precipitate on a rubber-tipped stirring rod or remove it by means of a watch 

 glass to a small evaporating dish and dissolve it hi a little water. To remove the 

 ammonium sulphate, which adhered to the precipitate and is now hi solution, 

 add barium carbonate, boil, and filter off the precipitate of barium sulphate. 

 Concentrate the proteose solution to a small volume 2 and make the follow- 

 ing tests : 



(1) BiuretTest. 



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

 indicate? 



(3) Precipitation by Trichloracetic Acid. This precipitate dissolves on 

 heating and returns on cooling. 



(4) Precipitation by Picric Acid. This precipitate also disappears on heat- 

 ing and returns on cooling. 



(5) Precipitation by Potassio -mercuric Iodide and Hydrochloric Acid. 



(6) Coagulation Test. Boil a little hi a test-tube. Does it coagulate? 



(7) Acetic Acid and Potassium Ferrocyanide Test. 



The solution containing the peptones should be cooled and filtered, and the 

 ammonium sulphate hi 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 and repeat the tests as given under the 

 proteose solution, above. Also try the precipitation by phosphotungstic acid 

 and by tannic acid. In the biuret test the solution should be made very strongly 

 alkaline with solid potassium hydroxide. 



PEPTIDES 



The pep tides are "definitely characterized combinations of two or 

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



1 The separation of proteoses and peptones by means of fractional precipitation with 

 ammonium sulphate does not possess the significance it was once supposed to possess inas- 

 much as the boundary between these substances undfeptidesiz not well defined (see p. 117). 



8 If the proteoses are desired in powder form, this concentrated proteose solution may- 

 now be precipitated by alcohol, and this precipitate, after being washed with absolute 

 alcohol and with ether, may be dried and powdered. 



