THE SYNTHESIS OF THE PROTEINS 19 



Carbethoxyl-N-phenyl-glycyl-glycine ester on hydrolysis yielded 

 the acid 



HOOC . N(C 6 H 5 ) . CH 2 . C(OH) = N . CH a . COOH, 



which did not lose carbon dioxide, and was analogous to Fischer's 

 glycyl-glycine carboxylic acid. Phenyl-glycyl-glycine carboxylic acid 

 easily forms the lactone 



/ CH 2 V 



N(C 6 H B )( )C = N.CH 2 .COOH, 



\CO . O/ 



as do also its derivatives ; thus carbethoxyl-N-phenyl-glycyl-glycine 

 ester when treated with ammonia yields the amide 



C 2 H 5 . OC . N(C 6 H S ) . CH 2 . C(OH) = N . CH 2 . CO . NH 2 , 

 which loses alcohol at 220 and forms the lactone 



/ CH 2 . 



N(C G H 5 )/ \C = N.CH 2 .CO.NH 2 . 



\CO.O/ 



The stability of glycyl-glycine carboxylic acid obtained from carb- 

 ethoxyl-glycyl-glycine ester is therefore due to its having the lactim 

 formula, 



HOOC . NH . CH 2 . C(OH) = N . CH 2 . COOH, 



whereas the instability of glycyl-glycine carboxylic acid obtained from 

 glycyl-glycine and carbonic acid is due to the lactam formula, 

 HOOC . NH . CH 2 . CO . NH . CH 2 . COOH. 



Further proof for these formulae is given by Siegfried's experiments 

 in which he showed that the peptide linking in polypeptides, which have 

 the lactam formula, could also combine with carbonic acid, whereas 

 glycyl-glycine carboxylic acid which has the lactim formula did not 

 combine with carbonic acid. 



