PROTEINS 



479 



without difficulty. Oxyhemoglobin, egg and serum albumm, 

 certain vegetable proteins, and others are obtainable in the form 

 of well-defined crystals. 



Color Reactions. — Color reactions were formerly thought to be 

 characteristic of definite kinds of proteins, but we now know 

 that most of these reactions are due to special amino-acid groups. 

 The biuret reaction gives a red or violet color when an excess of 

 caustic soda and a trace of copper sulphate are added to a protein 

 solution. This reaction is given by all proteins, by proteoses, 

 peptones, and nearly all of the synthetic polypeptides, but the 

 test fails with individual amino acids. The reaction receives its 

 name from biuret (bi + urea), C2H5N3O2, a substance formed by 

 exposing urea to high temperatures : 



CONH2 



Nnh 



CONH2 



The formula of biuret suggests that the color reaction is given by 

 substances having two amino groups (NH2) in their molecule. 



Strong sulphuric acid added to a protein solution containing 

 alcoholic alpha-naphthol produces a violet (or red) color. The 

 reaction is given by all proteins that contain carbohydrate com- 

 plexes and depends upon the production of furfural. 



Coagulation. — Coagulation, in the broadest meaning of the 

 term, includes a variety of phenomena some of which may be 

 identical, some very similar, and some quite distinct, yet all are 

 called by this name. Coagulation takes place when blood comes 

 into contact with air, when rennin or bacteria are added to milk, 

 or when proteins are heated. These are true coagulation 



