POLYPEPTIDES 65 



An examination of the results of hydrolysis by trypsin shows that 

 several factors have an important influence : 



1. The Structure of the Molecule. 



Glycyl-alanine, NH 3 . CH, . CO NH . CH(CH,) . COOH, was not 

 hydrolysed, but the isomeric alanyl-glycine, NH, . CH(CH a ) . CO 

 NH . CH 3 . COOH, was hydrolysed ; again, alanyl-leucine A was hydro- 

 lysed but not leucyl-alanine. 



The order in which the amino acids are combined together in the 

 molecule has therefore a very marked effect Thus, when alanine is the 

 acyl radicle, as in alanyl-glycine, alanyl-alanine and alanyl-leucine, hy- 

 drolysis occurred, but the reverse happened when leucine, valine or amino- 

 butyric acid were the acyl radicles ; in the cases of leucyl-alanine and 

 leucyl-glycine, no hydrolysis took place ; here the racemic compound 

 was employed and the resistance might have been due to this factor, 

 but the instance of 1-leucyl-glycine appears to confirm the older result. 



If tyrosine, isoserine and cystine stood at the end of the chain, 

 trypsin hydrolysed the compound ; in the only case examined where 

 tyrosine, combined with yS-naphthalene-sul phonic acid, acted as the 

 acyl radicle, there was no hydrolysis. 



2. The Configuration of the Molecule. 



This is of the greatest importance in the hydrolysis of a polypeptide 

 by trypsin, as will be seen from the list of compounds published in 

 1907. Only those compounds containing the naturally occurring opti- 

 cally active variety of the amino acid are hydrolysed by trypsin. 



The compounds marked with an asterisk are racemic and their hy- 

 drolysis was effected asymmetrically, only that portion of the molecule 

 containing the natural stereoisomer being attacked. This explained 

 the difference between alanyl-leucine A and alanyl-leucine B ; the 

 former probably consisted of d-alanyl-1-leucine + 1-alanyl-d-leucine, the 

 first of which contains the natural stereoisomers upon which the 

 hydrolysis depends; the latter would consist of d-alanyl-d-leucine + 

 1-alanyl-l-leucine. The later experiments of 1907 proved this supposi- 

 tion. It is more noticeable in the case of leucyl-leucine, which must 

 have been 1-leucyl-d-leucine + d-leucyl-1-leucine since 1-leucyl-l-leucine 

 is hydrolysed. 



3. The Number of Amino Acids. 



Only the various polypeptides containing several glycine radicles 

 come at present under this heading. Several interesting details are at 

 once apparent. Hydrolysis first took place when five glycine radicles, 

 as in tetraglycyl-glycine, are combined together, although it occurred in 



PT. II. 5 



