THE PROTEINS 103 



nitrogen among the various classes of proteins. The method is, 

 however, only a rough one as compared with the separation of the 

 individual amino-acids. 



An improved means of determining the distribution of nitrogen 

 in the protein molecule has been devised by Van Slyke. Some of his 

 results are given in Table II., p. 102. 



TESTS FOR PROTEIN 

 A. COLOUR REACTIONS OF THE PROTEINS 



These are of importance since in many cases they are an 

 indication of the nature of the groups present in the protein 

 molecule. 



(1) THE BIURET REACTION. When a solution of a protein 

 is made strongly alkaline with caustic potash or soda, and dilute copper 

 sulphate added drop by drop, a colour varying from pink to violet 

 is produced. In the case of the proteoses and peptones (the hydrated 

 proteins) the colour is pink ; in the case of the coagulable proteins, 

 violet. According to Schiff this colour is given by all compounds 

 containing the following groups : 



CO.NH 2 



CO.NH 2 

 CO.NH 2 



CO.NH 2 

 CO NH 2 



CO NH 2 



and the group 



(NH 2 )C CO NH C 



We have already seen that this grouping is typical of the protein 

 molecule. 



(2) THE XANTHO-PROTEIC REACTION. On adding strong 

 nitric acid to a solution of protein and boiling, a yellow colour is pro- 

 duced which turns to a deep orange when excess of caustic alkali or 

 ammonia is added. The production of this reaction points to the 

 existence of benzene derivatives in the protein molecule, and it is 

 therefore a general test for the presence of aromatic groups. 



