THE CHEMISTRY OF THE PROTEINS 115 



their hydrolysis by trypsin depends upon several factors. The 

 influence of their structure is exemplified by alanylglycine which 

 is hydrolysed, the isomeric glycylalanine not being hydrolysed. 

 Similarly, alanylleucine A is acted upon, but not leucylalanine. 

 The influence of the individual amino acid is more marked ; 

 thus, where alanyl is the acyl radicle, hydrolysis takes place ; 

 but not when leucyl or aminoisovaler}d (valyl) are in this 

 position. Hydrolysis also occurs when tyrosine, serine, and 

 cystine stand at the end of the chain. 



The configuration of the molecule is also of importance ; 

 the racemic compounds are hydrolysed asymmetrically with the 

 formation of the optically active amino acid which occurs in the 

 proteins ; thus alanylleucylglycine gave d-alanine and leucyl- 

 glycine mixed with the other form originally contained in the 

 racemic tripeptide. The identification of the racemic form in 

 the case of the combinations of alanine and leucine by means of 

 trypsin has been previously mentioned. Alanylleucine A was 

 partially hydrolysed, and was supposed to be composed of the 

 naturally occurring amino acids. This has quite recently been 

 proved by the study of the action of trypsin upon polypeptides 

 built up entirely of optically active amino acids. Again, leucyl- 

 leucine was not attacked, and it is now shown that it is the 

 racemic compound, 1-leucyl-d-leucine and d-leucyl-1-leucine. 



The following list gives these recent results : 



Those Hydrolysed Those not Hydrolysed 



d-alanyl-d-alanine d-alanyl-1-alanine 



d-alanyl-1-leucine 1-alanyl-d-alanine 



1-leucyl-l-leucine 1-leucyl-glycine 



1-leucyl-d-glutamic acid 1-leucyl-d-leucine 



d-leucyl-1-leucine. 



The number of amino acids in the molecule, i.e. the length 

 of the chain, is very interesting in the case of the glycine 

 polypeptides. Not until five glycine radicles, as in tetraglycyl- 

 glycine, are present, is hydrolysis produced by trypsin. It is 

 curious to note that the ester of triglycylglycine is hydrolysed, 

 but not the free acid. This is the biuret base of Curtius, and 

 was previously examined by Schwarzschild in its behaviour to 

 trypsin. It is probable that the configuration of the dileucyl 

 group in dileucylglycylglycine prevents its hydrolysis by trypsin, 



