36 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



NH 2 . CH 3 . CH(NH 2 ) . CO NH . CH 2 . CH(NH 2 ) . COOH 

 NH 2 . CH 2 . CH(NH 2 ) . CO NH . CH . COOH 



CH 2 . NH 2 



The diketopiperazines which are so closely related to the dipeptides 

 can also occur in a keto- or -enol form ; the possibilities are 



CO-CH 2 C(OH)-CH 2 C(OH)-CH 2 



2 

 NH )NH N 



\CH CO/ \CH 2 C(OH^ \ CH CO 



The existence of the -enol form was emphasised by the fact that in 

 the hydrolysis of alanine anhydride by alkali, a transient formation of 

 an alkali compound was observed. 



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 therefore occur in 2 n forms. Thus, 

 a dipeptide, 



NH 2 . 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 forms, z>., eight, a tetrapeptide in 2 4 or 

 sixteen forms, etc. 



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

 optically inactive compounds 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, e.g., 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 stereoiso- 

 merism upon the combination of the compounds, which is especially 

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

 in the rate of combination of the two compounds, which was first ob- 

 served by Markwald and Mackenzie with simple compounds. The 

 latter explanation is the more probable, since when both compounds 

 have been isolated, their amounts have been very different. There 

 still remains the possibility that the single substance isolated is still a 



