56 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



molecule alanine, and one molecule tyrosine, i.e., a tetrapeptide. It 

 had a molecular weight determined by the freezing-point method of 

 about 350, was easily soluble in water, insoluble in alcohol, and was pre- 

 cipitated from its solution in flakes by saturation with ammonium 

 sulphate or sodium chloride, as also by nitric or acetic acid. The syn- 

 thetical pentapeptide 1-leucyl-triglycyl-l-tyrosine behaves in a similar way 

 so that great complexity, as formerly believed, is not an essential condi- 

 tion for precipitation by ammonium sulphate. By the action of trypsin 

 tyrosine was split off, and on partial hydrolysis glycyl-d-alanine anhy- 

 dride and glycyl-1-tyrosine anhydride were obtained. 



In the same year (1907) the products obtained from elastin by 

 partial hydrolysis were shown by Fischer and Abderhalden by the same 

 methods to contain 



1. d-Alanyl-1-leucine. The anhydride of this dipeptide was also 

 obtained ; it probably arose from this dipeptide, but it can also be 

 formed from the isomeric 1-leucyl-d-alanine, whose presence amongst 

 the products is not excluded. 



2. Alanyl-proline anhydride, from which d-alanine and proline were 

 obtained on hydrolysis. 



3. Glycyl-valine anhydride, which was identical in its properties, 

 except the melting-point, with the synthetical compound. 



From gliadin Fischer and Abderhalden have also isolated a dipep- 

 tide by these methods, namely, 1-leucyl-d-glutamic acid, which they 

 identified with the synthetical substance ; Abderhalden and Funk have 

 isolated leucinimide, 1-phenylalanyl-d-alanine anhydride by acid hydro- 

 lysis from casein, and probably also leucyl-valine anhydride. 



In addition to these dipeptides isolated by Fischer and Abderhalden, 

 Osborne and Clapp have obtained a crystalline substance by the acid 

 hydrolysis of gliadin, which yielded proline and phenylalanine on further 

 hydrolysis, and Levene and Beatty have isolated a dipeptide composed 

 of glycine and proline from the products resulting by a trypsin digestion 

 of gelatin. The exact nature of these bodies has still to be determined 

 by comparison and identification with the synthetical substances, and 

 not until this has been done can their presence in the molecule be 

 accepted with certainty. 



The appended list gives the polypeptides which have so far been 

 isolated from proteins and therefore of the combinations of amino acids 

 which occur in the protein molecule : 



Glycyl-d-alanine anhydride in silk-fibroin. 

 Glycyl-d-alanine in silk-fibroin. 

 Glycyl-1-tyrosine anhydride in silk-fibroin. 



