428 CHEMICAL DISCOVERY AND INVENTION 



amino-propionic acid CH 3 -CHNH 2 'CO-OH and leucine or amino- 

 caproic acid, CH 3 -CH 2 -CH 2 -CH 2 -CHNH 2 -CO-OH, have also been 

 obtained as common products of this kind of change. Tyrosine 

 has also long been recognised among these products ; it is 

 phenyl-p-hydroxy-a amino-propionic acid 



CH 2 -CHNH 2 -CO-OH. 



Other more complicated substances are also found among the 

 products of disintegration of proteins, but they almost all partake 

 of the same character and behave as amino-acids. 



These facts furnished the principal clue to the mystery of the 

 constitution of albuminoid substances generally, and led to the 

 remarkable achievements in the direction of their synthesis 

 which we owe chiefly to Emil Fischer and his school. The 

 assumption is that the peptones are constituted of amino-acid 

 groups, and that the number of such residues joined together in 

 the molecule of the protein is relatively small. This view has 

 been practically established by the production of a number of 

 compounds called by Fischer, potypeptides, which in many re- 

 spects resemble the natural peptones, and there seems every 

 reason to expect that further efforts on similar lines will lead to 

 the synthesis of some substances identical with a natural albumin. 



The simplest of the peptides is glycyl-glycine which, containing 

 the residues of two amino-acid groups, is called a dipeptide. 

 Others containing three, four, or many such residues are named 

 respectively tri-, tetra-, and poly-peptides. 



Glycyl-glycine is obtained as follows : a amino-esters, when 

 heated, part with the elements of alcohol and a closed-chain 

 anhydride is formed, thus : 



Two molecules of glycine ethyl ester 



NH*-CIL>-CO-OC 9 H S 



2(NH 2 -CH 2 -CO-OC 2 H 5 ) or | 



C 2 H 5 0-CO-CH 2 -NH 2 



yield, by loss of H from the NH 2 in one molecule and C 2 H 5 

 from the other molecule, 



anhydride alcohol 



