DETERMINATION OF THE STRUCTURE OF PROTEINS. 

 Isolation of Polypeptides from Proteins. 



The earlier work of Fischer and Abderhalden on the digestion of 

 proteins by trypsin, and by pepsin followed by trypsin, pointed to a 

 combination of proline and phenylalanine in a part of the molecule, 

 but the compound was too complex to give any real clue to the order of 

 their combination. The first exact information as to the order of the 

 arrangement of the amino acids was given by Fischer and Bergell in 1 902, 

 who described the formation of a dipeptide by the hydrolysis of silk- 

 fibroin. 1 



As is well known, silk-fibroin readily dissolves in cold concentrated 

 hydrochloric acid ; if alcohol be then added, a product, called sericoin 

 by Weyl, is precipitated, but if the silk-fibroin be allowed to stand in 

 contact with three times its quantity of concentrated acid for about 

 twenty-four hours, alcohol no longer produces such a precipitate, and 

 the solution contains the hydrochloride of a peptone. Fischer and 

 Bergell removed the hydrochloric acid from the solution of the pep- 

 tone and concentrated in vacua ; a mass was obtained which had a 

 bitter taste, was very soluble in water and gave strong biuret and 

 Millon reactions, and which was very like peptone in its properties. 

 It lost the whole of the tyrosine which it contained when it was dis- 

 solved in water and digested in ammoniacal solution with trypsin, 

 and was converted into another peptone composed of 40*1 per cent, 

 glycine and 28*5 per cent alanine. Ammonia was evolved when 

 this compound was heated with baryta water and the solution, 

 freed from baryta, yielded crystals on evaporation ; these were treated 

 with /9-naphthalene-sulphonylchloride, and a compound was obtained 

 which was apparently /9-naphthalene-sulphonyl-glycyl-alanine, though 

 it could not be absolutely identified with the synthetical product of 

 this composition. 



The further attempts to prepare this substance again did not suc- 

 ceed, since the exact conditions leading to its formation could not be 



1 A small quantity of a crystalline compound was isolated in 1849 by Bopp from the 

 products of hydrolysis of caseinogen ; this was shown to be leucinimide (p. 13) by comparison 

 with the condensation product obtained by heating leucine. Kohn's product was apparently 

 also leucinimide. 



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