THE. CUEMISTRY 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 léucyl or aminoisovaleryl (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, I-leucyl-d-leucine and d-leucyl-l-leucine. 
The following list gives these recent results: 
Those Hydrolysed Those not Hydrolysed 
d-alanyl-d-alanine d-alanyl-l-alanine 
d-alanyl-l-leucine ]-alanyl-d-alanine 
l-leucy]-l-leucine l-leucyl-glycine 
l-leucyl-d-glutamic acid l-leucyl-d-leucine 
d-leucy]-l-leucine. 
The number of amino acids in the molecule, z.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, 
