CHEMICAL COMPOSITION OF PROTEIN MOLECULE 63 



DIFFERENTIATION OF THE PROTEINS BY THE 



DISTRIBUTION OF THE VARIOUS KINDS 



OF NITROGEN. 



Since a complete analysis of a protein is still an impossibility, 

 owing to the unsatisfactory methods for isolating and estimating the 

 several monoamino acids, the proteins cannot yet be differentiated by 

 means of their chemical composition. 



It has been proved by Osborne, Leavenworth and Brautlecht that 

 the methods for estimating the ammonia content and the diamino acid 

 (the three hexone bases) content of a protein are almost perfect. We 

 can therefore differentiate proteins by their content in these four 

 products. 



The monoamino acids and tryptophane all contain an amino 

 group : proline, oxyproline and tryptophane contain a heterocyclic ring : 

 cystine contains sulphur. These differences can be made use of for a 

 further differentiation as was shown by Van Slyke in 1911. We can 

 ascertain the following particulars : 



I. Amide nitrogen (ammonia). 



' Diamino nitrogen [a cystine ~| contain only amino N. 

 b lysine j" difference 



Sulphur content gives cystine. Lysine by 



III. 



form. with alkali. Histidine 



by difference. 



aspartic acid, tryptophane () 

 glutamic acid. 



^Non-amino nitrogen c proline 



b \ oxyproline 

 I tryptophane (^) 



i.e., seven data out of the possible eighteen. If we regard the data for 

 tyrosine as almost accurate, we have still one more value of service for 

 the chemical differentiation of the proteins. It is unfortunate that we 

 cannot yet measure the trypotophane content of a protein, especially 

 as this unit is so readily detected by means of its colour reactions. 



