PROTEINS. 93 



OH OH 

 CO NH 2 Cu NH 2 -CO 



NH HN 



/ \ 



CO NH 2 K K NH 2 - CO 



OH OH 



6. Posner's Modification of the Biuret Test. This test is par- 

 ticularly satisfactory for use on dilute protein solutions, and is 

 carried out as follows : To some dilute egg albumin in a test- 

 tube add one-half its volume of potassium hydroxide solution. 

 Now hold the tube in an inclined position and allow some very 

 dilute cupric sulphate solution, made as suggested on page 92 (5), 

 to flow down the side, being especially careful to prevent the fluids 

 from mixing. At the juncture of the two solutions the typical end- 

 reaction of the biuret test should appear as a colored zone (see 

 Biuret Test, page 92). 



7. Liebermann's Reaction. Add about 10 drops of concen- 

 trated egg albumin solution (or a little dry egg albumin) to about 

 5 c.c. of concentrated HC1 in a test-tube. Boil the mixture 

 until a pinkish-violet color results. This color was originally sup- 

 posed to indicate the presence of a carbohydrate group in the pro- 

 tein molecule, the furfurol formed through the action of the acid 

 upon the protein reacting with the hydroxy-phenyl group of the pro- 

 tein producing the pinkish-violet color. It is now considered un- 

 certain whether the carbohydrate group enters into the reaction. 

 Cole has called attention to the fact that a blue color results if pro- 

 tein material which has been boiled with alcohol and subsequently 

 zvashed with ether be used in making the test. He believes the blue 

 color to be due to an interaction between the glyoxylic acid, which 

 was present as an impurity in the ether used in washing the protein, 

 and the tryptophane group of the protein molecule which was split 

 off through the action of the acid. 



8. Acree-Rosenheim Formaldehyde Reaction. Add .a few 

 drops of a dilute (i 15000) solution of formaldehyde to 2-3 c.c. of 

 egg albumin solution in a test-tube. Mix thoroughly and after 

 2-3 minutes carefully introduce a little concentrated sulphuric acid 

 into the tube in such a manner that the two solutions do not mix. 

 A violet zone will be observed at the point of juncture of the 



