28 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



Both the free acid and its ester have a great tendency to become 

 reconverted into glycine anhydride, and both compounds are char- 

 acterised by the great reactiveness of the NH 2 group ; thus, with 

 phenylisocyanate they both yield the compound 



C 8 H 6 .NH.OO.NH.CH,.CO.NH.CH,.COOH; 



and the ester on combination with ethyl chlorocarbonate gives 

 carbethoxyl-glycyl-glycine ester : 



Cl . CO . OC,H, + NH, . CH, . CO . NH . CH, . COOC,H 6 = 

 HC1 + C,H 8 O . OC . NH . CH, . CO NH . CH, . COOCaH g , 



from which the amide, 



C,H 8 O . OC . NH . CH,. CO NH . CH,. CONH* 



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



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

 by careful hydrolysis with soda. 



When carbethoxyl-glycyl-glycine ester was heated with leucine 

 ester combination occurred and the compound, carbethoxyl-glycyl- 

 glycyl-leucine ester, was formed : 



C,H,O . OC . NH . CH,. CO . NH . CH, . COOC 2 H 5 + NH, . CH(C 4 H,,) . COOC,H 6 = 

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



This compound 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 Scruff in 1900, who found that glycine amide 

 NH 2 . CH 2 . CO . NH a 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, were obtained from alanine anhydride, and leucyl-leucine 

 from leucine anhydride. 



The hydrolysis of the anhydrides of the higher amino acids is more 

 difficult to effect. The diketopiperazine ring is not easily split open 

 by means of acid, and although Fischer, in 1905, discovered that the 

 diketopiperazine could be easily converted into the dipeptide by 

 treatment with the equimolecular quantity of caustic soda in 10-15 

 minutes at the ordinary temperature in the cases of glycyl-glycine 

 and alanyl-alanine, yet in other cases, such as that of leucine anhy- 

 dride, the anhydride was very resistant to alkali ; in this case hydro- 

 lysis was best effected by hydrobromic acid. It appears that the 

 stability of the diketopiperazine ring is connected with the nature of 



