DETERMINATION OF STRUCTURE OF PROTEINS 85 



in silk-fibroin. 



After two years of work upon this substance Abderhalden ulti- 

 mately succeeded in isolating I gramme of this tripeptide in a pure con- 

 dition. It was identified as d-alanyl-glycyl-1-tyrosine, and it is the 

 first tripeptide which has been isolated from the products of the partial 

 hydrolysis of proteins. 



The appended list gives the polypeptides which have so far been 

 isolated from proteins and therefore of the order in which the amino 

 acids occur in the proteins from which they have been isolated : 



Glycyl-d-alanine anhydride 



Glycyl-d-alanine 



Glycyl-1-tyrosine anhydride 



Glycyl-1-tyrosine 



d-Alanyl-glycine 



d-Alanyl-1-serine anhydride (?) 



d-Alanyl-glycyl-1-tyrosine 



Tetrapeptide (2 glycine -t- i alanine + i tyrosine) 



Glycyl-1-leucine anhydride 



d-Alanyl-1-leucine anhydride 



d-Alanyl-1-leucine 



1-Leucyl-d-alanine 



d-Alanyl-proline anhydride 



Glycyl-valine anhydride 



Leucyl-glycine 



Gycyl-leucine (?) 



Leucinimide 



1-Phenylalanyl-d-alanine anhydride 



1-Leucyl-d-valine anhydride 



1-Leucyl-d-glutamic acid 



I m gliadin. 

 1-Prolyl-l-phenylalanine / 



Prolyl-glycine anhydride in gelatin. 



Dipeptide (glutamic acid + tryptophane) ? 1 . 



I in edestm. 

 Tnpeptide (leucine + glutamic acid + tryptophane) ? ) 



The knowledge of the properties of the synthetical compounds has 

 been of the greatest service for the isolation of polypeptides from pro- 

 teins, especially the preparation of the anhydrides from the esters and 

 the naphthalene-sulphonyl derivatives, but at present no systematic 

 way of separating polypeptides from the mixture of complex products 

 arising on partial hydrolysis exists. The mixture is usually divided 

 into two fractions by precipitation with phosphotungstic acid. Each 

 fraction, after removal of the reagent, is then fractionally crystallised 

 or it may be treated with some other reagent, such as mercuric chloride. 

 The identification of any product is also beset with many difficulties ; 

 its elementary composition, molecular weight and the products it yields 

 on hydrolysis are not sufficient to establish its identity ; its physical 

 properties must be compared with those of the synthetical compound 

 and, if necessary, derivatives may require preparation. In the case of 



in elastin. 



in casemogen. 



