50 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



3. /ttwtr-leucine anhydride from d-leucyl-1-leucine ester and am- 

 monia and from 1-leucyl-d-leucine ester and ammonia. 



Hydrolysis of these anhydrides by alkali should give the corres- 

 ponding dipeptide, but in the case of the aminobutyryl-aminobutyric 

 acid anhydrides Fischer and Raske have found that a steric rearrange- 

 ment occurred ; the dipeptide A (p. 47) was obtained both from the anhy- 

 dride A and B ; these had been prepared by the action of ammonia on 

 the respective inactive esters, and in this manner the dipeptide B can 

 be converted into the dipeptide A. 



The number of isomers of diketopiperazines containing two mole- 

 cules of different amino acids is greater than when the two amino acids 

 in the molecule are the same. It can be calculated from the number 

 of asymmetric carbon atoms in the molecule just as in the open chain 

 compounds ; thus alanyl-leucine anhydride can exist in four optically 

 active forms and two racemic forms : 



1 . d-alanyl-d-leucine anhydride ; 



2. d-alanyl-1-leucine 



3. 1-alanyl-d-leucine 



4. 1-alanyl-l-leucine 



5. a mixture of I and 4 ; 



6. a mixture of 2 and 3. 



The same diketopiperazines are obtainable either from the dipep- 

 tides alanyl-leucine or from the dipeptides leucyl-alanine, so that in 

 fact the number of isomers of a diketopiperazine composed of two 

 different amino acids is less than those of the isomers of the dipeptide 

 composed of the same two amino acids. 



At present only a racemic form prepared from leucyl-alanine by 

 fusion has been obtained ; its nature has not been determined. 



