448 PROTEINS 



latter is then allowed to act upon the amino group of a second 

 acid as follows : — 



R2 R2 



R1CHNH2COCI + NHgCH . COOH = R1CHNH2CONH . CHCOOH + HC 



The resulting polypeptide may be of considerable complexity, 

 according to the nature of R^ and Rg. 



By these and similar methods, employing other combina- 

 tions of amino acids, polypeptides containing a great many 

 different groupings have been synthesized. The one with the 

 longest chain as yet obtained is an octodecapeptide leucyltri- 

 glycyl-leucyltriglycyl-leucyloctoglycyl-glycine of the formula 



NH2CHCiH9CO[NHCH2CO]3NHCHCiH9CO[NHCH2CO]3NHCHC4H9CO[NHCH2CO]8 



NHCHjCOOH 



The more complex of these polypeptides resemble the proteins 

 in being colloidal substances which give the biuret reaction, 

 and in being precipitated from solution by phosphotungstic or 

 tannic acids and by ammonium sulphate. 



The action of digestive ferments upon them has been studied 

 by Abderhalden and others ; they are not readily attacked by 

 pepsin, but are hydrolysed by pancreatic or intestinal juice. 



A striking confirmation of Fischer's view concerning the 

 close connection existing between the polypeptides and the 

 natural proteins is to be found in the fact that the hydrolysis 

 of proteins, under suitable conditions, yielded four substances 

 which could be identified with synthetic polypeptides. Thus, 

 a solution of silk fibroin in hydrochloric acid was allowed 

 to stand for several days ; on evaporating, a residue was ob- 

 tained which, when digested with trypsin, yielded a peptone- 

 like substance ; the latter on hydrolysis with barium hydroxide 

 gave glycylalanine, which was identified by its naphthaline 

 sulphonic acid derivative.* Subsequently, the hydrolysis was 

 repeated under somewhat altered conditions, with the same 

 result that glycylalanine was obtained. f In a later communi- 

 cation, the same authors described the isolation of glycyl- 

 tyrosine from the products of hydrolysis of silk fibroin, and 



* Fischer and Abderhalden : " Ber. deut. chem. Gesells.," 1906, 39, 

 752. 



t Ibid., 2315. 



