PROTEINS 173 



which reference has been made in the preceding chapter, namely, 

 phosphotungstic, phosphomolybdic, tannic, picric, ferrocyanic, 

 and trichloracetic acids, the double iodide of potassium, mercuric 

 iodide, etc. The precipitates produced by strong mineral acids 

 are often soluble in excess of the acid, with the formation of certain 

 so-called " derived proteins," which are probably products of 

 the partial hydrolysis of the protein. 



The proteins are also precipitated out of solution by the addi- 

 tion of small amounts of salts of various heavy metals, such as the 

 chlorides, sulfates, and acetates of iron, copper, mercury, lead, etc. 

 This precipitation is different than that caused by the saturation 

 of the solution with the salts of the alkali metals, as in this case the 

 metal unites with the protein to form definite, insoluble salts, 

 which cannot be redissolved except by treatment with some 

 reagent which removes the metal from its combination with the 

 protein (hydrogen sulfide is commonly used for this purpose). 



(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 par- 

 ticular 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. 



(6) Millon's Reaction. A solution of mercuric nitrate con- 

 taining some free nitrous acid (Millon's reagent) produces a pre- 

 cipitate which turns pink or red, whenever it is added to a solution 

 which contains tyrosin, or a tyrosin-containing protein. 



(c) Xanthoproteic Acid Reaction. This is the familiar yellow 

 coloration which is produced whenever nitric acid comes in con- 

 tact with animal flesh. It is caused by the action of nitric acid 

 on tyrosin. 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 



