DETERMINATION OF STRUCTURE OF PROTEINS 81 



repeated, but in 1906 Fischer and Abderhalden obtained the anhydride 

 of this body by a new method which they had discovered for isolating 

 such compounds when mixed with amino acids and higher polypeptides. 

 This method depends upon the different behaviour of the esters of 

 these compounds ; l the esters of the simple mono-amino acids are easily 

 volatile in vacua and are therefore easily removed ; the esters of the dipep- 

 tides are not volatile and are converted by the action of ammonia into 

 their anhydrides or diketopiperazines, which crystallise readily and are 

 therefore easily separated from the esters of the higher polypeptides. 

 They thus obtained a methyl diketopiperazine, 



CH, CO 



NH NH 



\ / 



CO CH(CH 3 ) 



which yielded glycine and d-alanine on hydrolysis and which was 

 identical with a synthetical product prepared from glycine and d-alanine. 

 It resulted by the hydrolysis with 70 per cent, sulphuric acid followed 

 by trypsin, and by the hydrolysis with hydrochloric acid. 



This diketopiperazine could arise both from glycyl-d-alanine and 

 d-alanyl-glycine by the above method. It could also arise by synthesis 

 from glycine and d-alanine under the conditions of the experiment ; 

 control experiments were carried out to determine this possibility, and 

 they showed that this was impossible, so that there was no doubt con- 

 cerning the presence of a dipeptide amongst the products of hydrolysis. 

 Since this dipeptide was resistant to trypsin it was most probably 

 glycyl-d-alanine and not d-alanyl-glycine, which is easily hydrolysed by 

 this enzyme. 



At the same time another diketopiperazine, glycyl-1-tyrosine anhy- 

 dride, was also obtained ; its nature was established a little later by 

 identification with synthetical glycyl-1-tyrosine anhydride prepared from 

 the ester of chloracetyl tyrosine and ammonia. In one experiment its 

 yield amounted to 4-2 per cent, of the silk-fibroin employed. 



In the same way by the hydrolysis of elastin with 70 per cent, 

 sulphuric acid and by the action of ammonia upon the esters, a product 

 was isolated which was composed of glycine and 1-leucine and which 

 was identical with synthetical glycyl-1-leucine anhydride. 



In 1907 Fischer and Abderhalden definitely showed that the first 



1 Abderhalden and Hanslian have shown that polypeptides are not hydrolysed during 

 the esterification process if water be carefully excluded, but proteins are hydrolysed by 

 boiling with alcohol and hydrochloric acid. 



PT. II. 6 



