(2) Color reactions.--Certain specific groups which are present in most proteins give definite color reactions with various reagents. It is apparent that any individual protein will respond to a particular color reaction, or will not do so, depending upon whether the particular group which is responsible for the color in question is present in that particular protein. Color reactions to which most of the common plant proteins respond are the following ones: (_a_) _Biuret Reaction._--Solutions of copper sulfate, added to an alkaline solution of a protein, give a bluish-violet color if the substance contains two, or more, --CONH-- groups united together through carbon, nitrogen, or sulfur atoms. Inasmuch as most natural proteins contain several such groups, the biuret reaction is a very general test for proteins. (_b_) _Millon's Reaction._--A solution of mercuric nitrate containing some free nitrous acid (Millon's reagent) produces a precipitate which turns pink or red, whenever it is added to a solution which contains tyrosine, or a tyrosine-containing protein. (_c_) _Xanthoproteic Acid Reaction._--This is the familiar yellow coloration which is produced whenever nitric acid comes in contact with animal flesh. It is caused by the action of nitric acid on tyrosine. The color is intensified by heating, and is changed to orange-red by the addition of ammonia. (_d_) _Adamkiewicz's Reaction._--If concentrated sulfuric acid be added to a solution of a protein to which some acetic acid (or better, glyoxylic acid) has previously been added, a violet color is produced. This color will appear as a ring at the juncture of the two liquids, if the sulfuric acid is poured carefully down the sides of the tube, or throughout the mixture if it is shaken up. It depends upon the interaction of the glyoxylic acid (which is generally present as an impurity in acetic acid) upon the tryptophane group, and is therefore given by all proteins which contain tryptophane. (_e_) _Molisch's reaction_ for furfural will be shown by those proteins which contain a carbohydrate group. In applying this test, the solution to be tested is first treated with a few drops of an alcoholic solution of [alpha]-naphthol, and then concentrated sulfuric acid is poured carefully down the sides of the test-tube. If carbohydrates are present, either free or as a part of a protein molecule, a red-violet ring forms at the juncture of the two liquids. (_f_) _Sulfur Test._--If a drop of a solution of lead acetate be added to a solution containing a protein, followed by sufficient sodium hydroxide solution to dissolve the precipitate which forms, and the mixture is heated to boiling, a black or brown coloration will be produced if the protein contains cystine, the sulfur-containing amino-acid. FOOTNOTES: