THE PROTEIN SUBSTANCES. /I 



appears. The absorption spectra from pure biuret and egg 

 albumin solution are the same, which shows that the albumin 

 must split off this group under the influence of the alkali used. 

 The reaction is one of extreme delicacy and may be employed 

 for the recognition of traces of protein compounds. It is 

 used especially in the detection of peptone, one of the derived 

 protein substances. Derivatives of simpler nature, that is, 

 the products of the decomposition of proteins, do not give the 

 reaction. It is therefore of value in following the course of 

 experiments on the digestion or hydrolysis of proteins, as the 

 reaction disappears with the breaking down of the last protein 

 complex. 



Xickel salts exhibit an analogous behavior, but show orange 

 yellow colors. Cobalt solutions give reddish colors, but not 

 very strong. 



Ex. Prepare a dilute white of egg solution and add to 5 cc. of it some 

 solution of potassium or sodium hydroxide. Then add one or two drops 

 of weak copper sulphate solution, or enough to impart the characteristic 

 color. An excess of the copper yields a precipitate and must be avoided. 

 The reaction is much sharper with albumose and peptone derivatives than 

 with the original native protein. Repeat the test with solution of nickel 

 sulphate. The test is obscured by the presence of ammonium salts, which 

 is sometimes a matter of importance in practical work. 



The a-Naphthol Test of Molisch. In the chapter on the 

 sugars it was shown that a very marked color reaction is 

 given by mixing a few drops of a weak alcoholic solution of 

 a-naphthol with the sugar solution and then adding some 

 strong sulphuric acid. The same behavior is shown by solu- 

 tions of some protein substances, which indicates that they 

 must contain a carbohydrate group of some kind. The reac- 

 tion depends on the formation of furfuraldehyde by the decom- 

 position of the sugar by the strong acid. This furfuraldehyde 

 combines then with the a-naphthol to produce a deep violet 

 color, the reaction being similar to that between furfuralde- 

 hyde and aniline acetate described in the pentose test in a 

 former chapter. 



