PROTEINS 97 



obtained in crystalline form, notably egg albumin, serum albumin, and 

 lactalbumin, but the fact that they may be obtained in crystalline form 

 does not necessarily prove them to be chemical individuals. 



GENERAL COLOR REACTIONS OF PROTEINS 



These color reactions are due to a reaction between some one or 

 more of the constituent radicals or groups of the complex protein mole- 

 cule and the chemical reagent or reagents used in any given test. Not 

 all proteins contain the same groups and for this reason the various color 

 tests will yield reactions varying in intensity of color according to the 

 nature of the groups contained in the particular protein under examina- 

 tion. Various substances not proteins respond to certain of these color 

 reactions, and it is therefore essential to submit the material under ex- 

 amination to several tests before concluding definitely regarding its 

 nature. 



TECHNIC OF THE COLOR REACTIONS 



i. Millon's Reaction. To 5 c.c. of a dilute solution of egg albumin 1 in a test- 

 tube add a few drops of Millon's reagent. A white precipitate forms which turns 

 red when heated. 



This test is a particularly satisfactory one for use on solid proteins, 

 in which case the reagent, diluted with water 1:5, is added directly 

 to the solid substance and heat applied, which causes the substance to 

 assume a red color. Such proteins as are not precipitated by mineral 

 acids, for example certain of the proteoses and peptones, yield a red 

 solution instead of a red precipitate. 



The reaction is due to the presence of the hydroxy-phenyl group, 

 CeH 4 OH, in the protein molecule and certain non-proteins such as 

 tyrosine, phenol (carbolic acid) and thymol also respond to the reaction. 

 Inasmuch as the tyrosine grouping is the only hydroxyphenyl grouping 

 which has definitely been proven to be present in the protein molecule it 

 is evident that protein substances respond to Millon's reaction because * 

 of the presence of this tyrosine complex. The test is not a very satis- 

 factory one for use in solutions containing inorganic salts in large 

 amount, since the mercury of the Millon's reagent 2 is thus precipitated 

 and the reagent rendered inert. This reagent is therefore never used 

 for the detection of protein material in the urine. If the solution under 



1 This egg albumin solution may be prepared by beating egg-white with 6-10 volumes of 

 water. The precipitate of ovoglobulin is filtered off and the filtrate used in the tests. 



1 Millon's reagent consists of mercury dissolved in nitric acid containing some nitrous 

 acid. It is prepared by digesting one part (by weight) of mercury with two parts (by 

 weight) of HNOj (sp. gr. 1.42) and diluting the resulting solution with two volumes of 

 water. 



