POLYPKPTIDES 49 



The Configuration of the Anhydrides. 



Diketopiperazines composed of two molecules of the same amino 

 acids, *>., containing the same substituting group, according to theory, 

 can exist in four forms, which are comparable to those of the tartaric 

 acids, namely : 



1. The active dextro form, containing the two dextro rotating 

 molecules. 



2. The active laevo form, containing the two laevo rotating mole- 

 cules. 



3. The inactive form, a mixture of I and 2. 



4. The inactive form, containing a dextro and a laevo rotating 

 molecule. 



The following are the structural formulae for the anhydrides of 

 alanine : 



CH, CH, H H 



1/ NH - CO \J i/ NH - co \J 



I \CO NH/ I I \CO NH/ I 



H H CH, CH 3 



H CH S 



.NH CO. 



C \ T* 



\CO NH/ 



The substituting groups in the optically active forms are in the 

 V-position, in the inactive meso form in the trans- position. 



Fischer and Raske in 1906 showed that these various forms of the 

 alanine anhydrides existed They prepared the inactive trans-an- 

 hydride, 



1. by the action of ammonia upon 1-alanyl-d-alanine ester ; 



2. by the action of ammonia upon d-alanyl-1-alanine ester. 



The active d-alanine anhydride had been previously synthesised in 

 a similar way from d-alanyl-d-alanine ester, and an inactive anhydride 

 had been obtained by heating inactive alanine ester ; the latter probably 

 represents the inactive mixture of the dextro and laevo anhydrides ; 

 the remaining compound, 1-alanine anhydride, has not yet been prepared. 



The several forms of the diketopiperazines were synthesised in 1907 

 by Fischer and Koelker, who prepared : 



1. d-leucine anhydride from d-leucyl-d-leucine ester and ammonia ; 



2. 1-leucine anhydride from 1-leucyl-l-leucine ester and ammonia ; 



PT. II. 4 



