128 CHEMISTRY OF THE PROTEIDS CHAP. 



t 



He fell on the plan of protecting the NH 2 group by the introduc- 

 tion of a carboxethyl - radical ; and then uniting the carbethoxyl- 

 amino-acid in the form of its ester with other amino-acid-esters by 

 gentle heating, and so obtained carbethoxyl-glycyl-glycyl-leucin-ester. 

 Glycyl-glycin : 



NH 2 . CH 2 . CO NH . CH 2 . CO . OH 

 Glycyl-glycin-ester : 



NH 2 . CH 2 . CO NH . CH 2 . CO . OC 2 H 5 

 Carboxethyl-glycyl-glycin : 



H 5 C 2 . OC NH . CH 2 . CO NH . CH 2 . CO . OH 

 Carboxethyl-glycyl-glycin-ester : 



H 5 C 2 . OC NH . CH 2 . CO NH . CH 2 . CO . OC 2 H 5 

 Carbethoxyl-diglycyl-leucinester : 



C 2 H 5 . OC NHCH 2 CO . NHCH 2 CO NH . CH(C 4 H 9 )CO . OC 2 H 5 . 



As complex esters are less given to form condensation-products along 

 the line just laid down, Fischer converted ammo-acids into their 

 acid chlorides by adopting Hans Mayer's plan of preparing chlorides 

 of pyridin-carboxylic acid by means of thionyl-chloride. 1 The amino- 

 acids are first converted into carbethoxy-compounds, and the latter by 

 gentle warming with thionyl chloride into carbethoxy-amino-acid 

 chlorides, for thionyl chloride, SOC1 2 , acts on organic acids analogous 

 to PC1 5 or S0 2 C1 2 , giving acid chlorides ; for example, butyric acid + 

 thionyl chloride react thus : 



C 3 H 7 CO . OH + SOC1 2 - C 3 H 7 COC1 + S0 2 + HC1. 



As the chlorides of carboxethyl-amino-acids readily unite with the 

 esters of the same or other amino-acids, according to the equation, 

 H 5 C 2 . OC NH . CH 2 CO Cl + NH 2 CH 2 CO . OC 2 H 5 = 



Carboxethyl-glycin-chloride + glycin-ester = 



H 5 C 2 . OC NHCH 2 CO NHCH 2 CO . OC 2 H 5 



carboxethyl-glycyl-glycin-ester. 



Fischer found it easy to prepare the esters of carbethoxyl-glycyl-glycin 

 and of di-glycyl-glycin. The latter has the formula : 



(C 2 H 5 OOC) . NH . CH 2 CO NHCH 2 CO NHCH 2 CO . OC 2 H 5 . 



By repeating the process with the products of synthesis, such as the 

 dipeptid of glycocoll, it is possible to build up compounds con- 

 taining 4 glycocoll-radicals in the form of anhydrides or carbethoxy- 

 triglycyl-glycin-ester : C 2 H 5 O . OC [NHCH 2 CO] 4 OC 2 H 5 . 



By saponifying these compounds the corresponding acids are 

 obtained, while treatment with ammonia converts one of the ester- 

 groups into an amide : 



1 Hans Mayer, Monatshefte f. Ghent. 22. 11 (1901). 



