6 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



This compound contained two amino acid radicles joined together and 

 was called glycyl-glycine, the group 



NH,.CH,.CO 

 being termed the glycyl group ; it was the first dipeptide known. 



The anhydrides, or diketopiperazines, of alanine and of leucine 

 were afterwards prepared, and both gave on hydrolysis the correspond- 

 ing dipeptides, alanyl-alanine and leucyl-leucine. 



Difficulties were, however, experienced with the preparation of the 

 anhydrides of other amino acids, but a few years later he found that 

 they could be readily obtained by heating the esters of the amino acids 

 to 150-180 C. and that the diketopiperazine ring was very rapidly split 

 by the action of dilute alkali at the ordinary temperature. The anhy- 

 drides and dipeptides of alanine, leucine, phenylalanine and tyrosine 

 were thus obtained. The dipeptides of the diamino acids, lysyl-lysine 

 and histidyl-histidine, and of serine have up to the present time only 

 been made by this method. The method does not lend itself to 

 the preparation of anhydrides or dipeptides containing two different 

 amino acids. These compounds can, however, be prepared indirectly 

 (p. 29> 



The properties of the amino acids recur in the dipeptides : they are 

 easily converted into their esters, and higher polypeptides are formed 

 when these esters are heated. 



2. By introducing an a-halogen acyl radicle into the molecule of 

 an amino acid and then treating this compound with ammonia, the 

 halogen atom is replaced by the amino group and a dipeptide is 

 formed thus : 



Chloracetyl tyrosine is obtained by the action of chloracetyl chloride 

 upon tyrosine : 



Cl . CH, . COCl + NH, . CH . CH 8 . C 9 H 4 OH = HCl + Cl . CH, . CO NH . CH . CH, . C,H 4 OH 



COOH COOH 



From this compound glycyl-tyrosine is formed by the action of am- 

 monia : 

 C1.CH,.CO NH. CH . CH,. C 6 H 4 OH + NH,= HC1+ H,N . CH,. CO NH . CH . CH,. C 8 H 4 OH 



COOH COOH 



If this reaction be repeated, using a dipeptide in place of an amino 

 acid, a tripeptide results ; a tetrapeptide using a tripeptide and so on. 



Using various halogen acyl chlorides a large number of di-, tri-, 

 tetra-, and penta-peptides have been prepared. The method has its 

 limitations as all the corresponding halogen acyl radicles are not 



