320 PROTEINS 



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 



NHCH 2 COOH 



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 connexion existing between the polypeptides and the 

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

 of proteins, under suitable conditions, has yielded four sub- 

 stances 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 sul- 

 phonic 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 

 of glycyl-leucine from elastin. Levene and Beatty J also 

 claim to have obtained prolyl-glycine from the hydrolysis of 

 gelatine. 



Abderhalden also mentions certain substances of a poly- 

 peptide nature which he found amongst the products of pan- 

 creatic digestion of a number of proteins such as caseine, 

 edestine, haemoglobin, serum globulin, egg albumin and 

 fibroin. 



* Fischer and Abderhalden : " Ber. deut. chem. Gesells.," 1906, 39, 752. 

 f Fischer and Abderhalden : " Ber. deut. chem. Gesells.," 1906, 39, 2315. 

 Levene and Beatty: " Ber. deut. chem. Gesells.," 1906, 39, 2060. 

 Abderhalden : " Zeit. physiol. Chem.," 1905, 44, 28, 33. 



