90 THE PROTEIN SUBSTANCES. 



certain conclusions as to the structure of the peptides by comparing the hydro- 

 lytic products before and after deamidation. 



A comparison of the artificially prepared polypeptides with the pro- 

 teins, and especially with the cleavage products of these last, the so-called 

 preoteoses and peptones, is of great interest in several respects, especially 

 in connection with certain reactions. For instance there are several 

 polypeptides which give the biuret reaction which is characteristic of 

 the proteins in general, and also several (polypeptides containing tyrosine), 

 which give MILLON'S reaction (see further on). The above-mentioned 

 octadecapeptide is precipitated by phosphotungstic acid, tannin and 

 ammonium sulphate; we also know tri- and pentapeptides containing 

 tyrosine, which are very similar in properties to the proteoses. 

 p The behavior of the polypeptides with proteolytic enzymes is of 

 great interest. As this interesting question will be thoroughly treated 

 in other chapters (I and VIII) it is sufficient here to recall that the 

 possibility that polypeptides as well as proteins are hydrolyzed by the 

 same enzymes, yielding amino-acids, is a weighty proof of the prob- 

 ability that in the proteins the amino-acid chains are of the same kind 

 as in the polypeptides. 



A very important support for such a view is found in the occurrence 

 of polypeptides among the cleavage products of proteins, a find which 

 to a certain extent forms the reverse of the above-mentioned syntheses. 

 Such polypeptides are chiefly di- but also tri- and tetrapeptides. They 

 have been obtained in the hydrolytic products of silk waste, silk fibroin 

 and elastin (FISCHER, ABDERHALDEN) , gelatin (LEVENE, WALLACE and 

 BEATTY) and of gliadin (OSBORNE and CLApp) 1 . Of .special interest in 

 this connection are those polypeptides which like glycyl-d-alanine, 

 d-alanyl-glycine, glycyl-Z-tyrosine, /-prolyl-Z-phenylalanine and d-alanyl- 

 glycyl-/-tyrosine, are identical with the corresponding synthetically pre- 

 pared polypeptides or at least very closely related. 



We have therefore conclusive reasons for the assumption that in the 

 proteins, peptide bindings chiefly occur, i.e., a combination of the a- 

 amino-acids by means of the imide binding. It is also possible that 

 other linking may occur, and FISCHER has also given expression to such 

 a possibility. Besides the above-mentioned imide binding another kind 

 must also without doubt exist in the proteins, namely, the anchoring 

 of the urea-forming group (the guanidine residue) with the ornithin 

 (diamino-valeric acid) by the imide binding. This imide linking is 



1 Fischer and Abderhalden, Sitz. Ber. d. d. Berl. Akad. d. Wissensch, 30, and Ber. 

 d. d. Chem. Gesellsch., 39, 40; Abderhalden, Zeitschr. f. physiol. Chem., 62, 63 and 72; 

 Levene and Wallace, ibid., 47, with Beatty, Ber. d. d. Chem. Gesellsch., 39, and Bioch. 

 Zeitschr., 4; Osborne and Clapp, Amer. Journ. of Physiol., 18. 



