THE CHEMISTRY OF THE PROTEINS 113 



with more than one asymmetric carbon atom are represented 

 as follows : 



d-alanyl-d-leucinel t d-alanyl-1-leucine Itt 



1-alanyl-l-leucine J 1-alanyl-d-leucine J 



but as their configuration must be first established, Fischer has 

 simply labelled them A and B, the A denoting the more insoluble. 

 The synthesis of the optically active forms and their combination 

 together would definitely decide the constitution, but a simpler 

 method has been found by studying the action of trypsin upon 

 them. The A combination was hydrolysed ; d-alanyl-1-leucine 

 contains the natural amino acids, and hence the form II. most 

 probably corresponds to the more insoluble racemic dipeptide. 



The configuration of the diketopiperazines is far more 

 complex and need not be entered into here. 



Although the amino acids are in all probability joined 

 together in the manner of acid amides, it is possible that other 

 couplings may also be present in the protein molecule. Ethers 

 of the oxyamino acids, their esters with other amino acids are 

 conceivable ; and polyoxyamino acids, if present, suggest a still 

 more complex combination. The presence of diketopiperazine 

 rings is also likely, as they are so easily converted by alkali into 

 the dipeptides ; a change of this kind may take place in the 

 formation of the so-called albuminates and on coagulation. 

 The hydrolysis of the polypeptides by the proteolytic enzymes, 

 and their giving of the red biuret reaction like peptone, point 

 most strongly to the acid amide manner of combination of the 

 amino acids in the protein molecule. 



IV. The Action of Enzymes upon the Polypeptides 



We have seen that, by synthesis, a very large number of 

 polypeptides have been prepared from the amino acids ; their 

 number is almost infinite, even if only the optically active forms, 

 as they occur in nature, be coupled together. Some light might 

 be shed upon what combinations actually occur in the protein 

 molecule by the study of the products of hydrolysis, when this 

 is not carried to the ultimate stage. Such intermediate products 

 are already known and designated proteoses and peptones. 

 They consist of mixtures of complex polypeptides, and at 

 present our methods are not sufficiently perfect for separating 

 polypeptides from one another and from amino acids. They 



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