THE SYNTHESIS OF THE PROTEINS 23 



the great reactiveness of the NH 2 group ; thus, with phenylisocyanate 

 they both yield the compound C 6 H 5 . NH . CO . NH . CH 2 . CO . NH . 

 CH 2 . COOH, and the ester gives with ethyl chlorocarbonate, carbethoxyl- 

 glycyl-glycine ester : 



OC 2 H 5 

 C _ o + NH 2 . CH 2 . CO . NH . CH 2 . COOC 2 H 5 = C 2 H 5 . O . OC . NH . CH 2 . CO . NH . CH 2 . COOC 2 H 6 , 



Cl 



from which the amide 



C 2 H 5 . O . OC . NH . CH 2 . CO . NH . CH 2 . CONH a 



is obtained by the action of ammonia, and the free acid 



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



by careful hydrolysis with soda. 



Carbethoxyl-glycyl-glycine ester, when heated with leucine ester, 

 yielded carbethoxyl-glycyl-glycyl-leucine ester, 



C 2 H 5 . OC . NH . CH 2 . CO . NH . CH 2 . COOC 2 H 5 + NH 2 . CH(C 4 H 9 ) . COOC 2 H 5 = 

 C 2 H 5 OH + C 2 H 5 O . OC . NH . CH 2 . CO . NH . CH 2 . CO . NH . CH(C 4 H 9 ) . COOC 2 H 5 , 



a compound which contains three amino acids combined together and 

 was the first known representative of a tripeptide. 



Carbethoxyl-glycyl-glycine amide and carbethoxyl-glycyl-glycyl- 

 leucine ester give the biuret reaction as would be expected from the 

 researches of Schiff in 1900, who found that glycine amide NH 2 . CH 2 . 

 CO. NH 2 also gave the reaction. 



In the same way alanyl-alanine and alanyl-alanine ester, which 

 yielded carbethoxyl-alanyl-alanine ester when treated with ethylchloro- 

 carbonate, can be obtained from alanine anhydride, and leucyl-leucine 

 from leucine anhydride or leucinimide, which was first obtained in 

 1849 by Bopp, and regarded as occurring in the protein molecule, by 

 hydrolysis with hydrobromic acid. 



The condensation together of other amino acids in this way by 

 heating their esters is accompanied by difficulties. The diketopiperazine 

 ring is not easily split open by means of acid, and although Fischer, in 

 1905, discovered that the diketopiperazine could be converted into the 

 dipeptide somewhat easily by treatment with the equimolecular quantity 

 of caustic soda in 10-15 minutes at the ordinary temperature, whereby 

 glycyl-glycine and alanyl-alanine could be easily prepared, yet, in other 

 cases, such as that of leucine anhydride, the anhydride was very resistant 

 to alkali. It appears that the stability of the diketopiperazine ring is 

 connected with the nature of the alkyl groups attached to it, and that 

 there is here another instance of steric hindrance, 



