54 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



The Polypeptides Present in the Proteins. 



The formation of a dipeptide by the hydrolysis of silk-fibroin was 

 first described by Fischer and Bergell in 1902. 



' 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. On concentration 

 in vacuo, when freed from hydrochloric acid, 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. 

 When dissolved in water and digested in ammoniacal solution with 

 trypsin, this peptone lost the whole of the tyrosine which it contained, 

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

 glycine and 28'5 per cent, alanine. From this compound, when heated 

 with baryta water, ammonia was evolved, and the solution, freed from 

 baryta, on evaporation yielded crystals; these were treated with 

 /8-naphthalene-sulphochloride, and a compound was obtained which was 

 apparently /3-naphthalene-sulpho-glycyl-alanine, though it could not be 

 absolutely identified with the synthetical product of this composition. 



The further attempts to again prepare this substance did not succeed, 

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

 but in 1906 Fischer and Abderhalden obtained the anhydride of this 

 body by a new method which they had discovered for isolating such 

 compounds when mixed with amino acids and higher polypeptides. This 

 method depends upon the different behaviour of the esters of these 

 compounds ; those of the simple mono-amino acids are easily volatile 

 in vacuo and are therefore easily removed, whereas those of the dipep- 

 tides are converted by the action of ammonia into their anhydrides 

 or diketopiperazines which crystallise readily and are therefore easily 

 separated from the esters of the higher polypeptides. They thus ob- 

 tained a methyl diketopiperazine, 



CH 2 CO 



/ \ 



NH NH 



\ / 



CO CH(CH 3 ), 



which was identical with a synthetical product prepared from glycine 

 and d-alanine, and which yielded glycine and d-alanine on hydrolysis. 



