24 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



The dipeptides of the oxy- and diamino acids have so far only been 

 prepared by this method by Fischer and Suzuki ; here, the methyl 

 esters of the amino acids were found to be most easily converted into 

 the anhydrides, and hydrolysis by alkali proceeded readily. The 

 compounds thus obtained can be seen from the accompanying 

 list. 



Several of the dipeptides are most readily prepared in this way, and 

 they have been employed in the synthesis of more complex polypep- 

 tides. The method, however, does not lend itself to the preparation of 

 higher polypeptides, but it will be observed that pentaglycyl-glycine 

 and another compound, probably octaglycine anhydride, have been 

 prepared by heating the methyl ester of diglycyl-glycine. The various 

 compounds isolated by Curtius and his pupils, such as glycine 

 anhydride and the biuret base, have been obtained from glycine 

 ester. 



It must be noted that anhydrides are also formed when the dipep- 

 tides, prepared by the other methods, are heated to their melting-points, 

 e.g.) leucyl-proline anhydride from leucyl-proline. 



Mixed anhydrides, as for example glycyl-alanine anhydride, cannot 

 be obtained by heating a mixture of the esters, where a complex 

 mixture would result, but they are easily prepared by the action of 

 ammonia upon the esters of the dipeptides. These compounds, of 

 which several have now been prepared, are of great importance as they 

 serve for the isolation of dipeptides from a mixture of polypeptides 

 and amino acids (see polypeptides isolated from proteins). On hydroly- 

 sis they yield a mixture of the two dipeptides, composed of the amino 

 acids of which they are built up. Thus glycyl-1-tyrosine anhydride 

 yielded glycyl-1-tyrosine and 1-tyrosyl-glycine. The latter compound 

 is the first example of a polypeptide containing the tyrosine radical as 

 the acyl group ; in all the other polypeptides in which tyrosine is 

 present it stands at the end of the chain. 



