POLYPEPTIDES 47 



The Configuration of the Polypeptides. 



Excepting glycine all the amino acids employed in the previous 

 syntheses contain an asymmetric carbon atom. According to the law 

 of Van't Hoff, the polypeptides will occur in 2" forms. Thus, a 

 dipeptide, 



NH, . CHR . CO NH . CHR . COOH, 



containing two asymmetric carbon atoms will be capable of existence 

 in the four active forms 



dd' 11' dl' Id' 



of which the two first and the two last together will form a racemic 

 compound. 



A tripeptide can exist in 2 3 or eight forms, a tetrapeptide in 2 4 

 or sixteen forms, etc. 



The two inactive forms of a dipeptide are obtained when the two 

 optically inactive units are coupled together by synthesis, and they 

 appear first in the form of the corresponding halogen derivative, 



Br . CHR . CO NH . CHR . COOH. 



A separation of the two racemic forms has been effected in certain 

 cases at this stage, eg., leucyl-phenylalanine, but in the majority of 

 cases only one product has been isolated. The formation of only one 

 product in the reaction may be due either to the influence of stereo- 

 isomerism upon the combination of the units, which is especially 

 noticeable when enzymes are concerned, or it may be due to a differ- 

 ence in the rate of combination, an observation first made by Markwald 

 and Mackenzie. The latter explanation is the more probable, since 

 when both compounds have been isolated their amounts have been 

 very different. There remains the possibility that the single substance 

 isolated is still a mixture of the two compounds, as it is so difficult to 

 separate a mixture of substances having similar and almost identical 

 properties. 



The two racemic compounds when they have been isolated are 

 distinguished by terming the more insoluble compound A and the 

 more soluble compound B. By the action of trypsin, which only hydro- 

 lyses the compound containing the naturally occurring amino acids, 

 it has been possible to determine what the combinations present in 

 these compounds are ; thus, as alanyl-leucine A was hydrolysed by. 

 trypsin it must contain d-alanyl-1-leucine, and the two racemic com- 

 pounds will be 



d-alanyl-l-leucine \ d-alanyl-d-leucinc i 



l-alanyl-d-leucine> 1-alanyl-Meucine ' 



