PROTEINS 165 



Precipitation Reactions. — Owing to the amphoteric nature of 

 proteins they form insoluble salts and are precipitated out of 

 solution by the salts of the heavy metals such as iron acetate, 

 copper sulphate, etc., which form with the protein an insoluble 

 precipitate. For this reason egg white is administered in cases 

 of poisoning with metal salts like mercury. This differs from the 

 precipitation out of solution by the ordinary saturated salt solu- 

 tions in that, with the heavy metals, a definite insoluble salt is 

 formed and the protein is altered or denatured. 



Color Reactions. — Certain groups or radicals in the protein 

 molecule combine with various reagents to give specific color re- 

 actions. The presence of these reactions then determines whether 

 this particular radical or grouping is present in the molecule. 

 The reactions are therefore really tests for certain amino-acid 

 groups rather than for proteins as a whole. Among the several 

 color reactions three are commonly used : 



1. Milton's reaction. A solution of mercuric nitrate with free 

 nitrous acid present, produces a reddish precipitate in the pres- 

 ence of ty rosin or a protein which contains it. 



2. Xanthoproteic reaction. Proteins containing tyrosin are turned 

 yellow by nitric acid. This color is intensified by heating and is 

 changed to an orange color on the addition of ammonia. This is. 

 the familiar yellow color which develops on the fingers of a labora- 

 tory worker who has been handling nitric acid. 



3. Biuret reaction. If a solution of copper sulphate is added to 

 an alkaline solution of a protein which contains two CO.NH 2 



H 



groups as in biuret, 0=C— N— C=0, a bluish-violet or lavender 



NH 2 NH 2 



color results. Since most proteins' contain such groups, the biuret 

 reaction can be used for them. 



Physical Properties— Indiffusibility. - -The proteins belong to 

 the class of bodies known as colloids (Chap. II). Although there 

 are some who think that the individual protein molecules are not 

 so very large but that they group together to form large colloidal 

 complexes, the majority of workers hold that the molecules are of 

 enormous size, being the largest molecules known. Some of the 

 molecular weights have been determined and then from their 



