38 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



cules of a racemic acid chloride, e.g., a-bromopropionyl chloride, three 

 isomeric optically active compounds can result, namely : 



d-bromopropionyl-d-bromopropionyl-cystine 



1-bromopropionyl-l-bromopropionyl-cystine 



d-bromopropionyl-1-bromopropionyl-cystine 



A yield of 7 1 per cent, of dibromopropiony 1-cystine was obtained by 

 Fischer and Suzuki and was apparently a definite substance. It was 

 therefore regarded as the dl-compound, ^ince its formation is indepen- 

 dent of the formation of the dd- or 11-compounds which most probably 

 would result in equal amounts. 



The Configuration of the 2, 5- Diketopiperazines. 



Diketopiperazines composed of two molecules of the same amino 

 acids, i.e., containing the same substituting group, can, according to 

 theory, 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 molecules. 



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



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

 cule, as can be seen from the structural formulae for the anhydrides of 

 alanine : 



CH 3 CH 3 H H 



/NH CCK /NH I 



C \ / C C \ 



\CO NH/ I \CO-NH 



H H CH 3 CH 3 



H CH 3 



/NH CO, 



C \ / C 



I \CO-NH/ I 



CH 3 H 



The substituting groups in the optically active forms are thus in the 

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



The proof of the existence of these various forms was commenced in 

 1906 by Fischer and Raske in the case of the alanine anhydrides. They 

 prepared the inactive trans-anhydride 



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 



