THE PROTEINS 93 



easily displaceable nitrogen. The remaining fluid, freed from ammonia is 

 precipitated with phosphotungstic acid. By this means all the diamino-ac'ids 

 and bases are thrown down. The nitrogen in the preciptate is determined 

 by Kjeldahl's method and is called diamino- or basic nitrogen. In the 

 remaining fluid, which contains mono-amino-acids, the total nitrogen, the 

 mono-amino-nitrogen, is determined by Kjeldahl's method. Table I., p. 92, 

 gives some of the results obtained in this manner, and shows that there are 

 considerable differences in the distribution of the different kinds of 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. 92. 



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 : 



X CO.NH 2 



NH/ 



X 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 r 

 to a solution of protein and boiling, a yellow colour is produced wh 

 to a deep orange when excess of caustic alkali or ammonia is 

 production of this reaction points to the existence of benzene den 

 the protein molecule, and it is therefore a general test for t 

 aromatic groups. 



