142 THE PROTEINS AND THE AMINO-ACIDS 



If the bromisocapronyl-glycyl chloride be made to act upon glycyl- 

 glycine ester, and the product treated with ammonia, the tetrapeptide 

 Leucyl-diglycyl-glycine results: 



C4H9.CH(NH 2 )CO.NHCH 2 .CO.NHCH 2 CO.NHCH 2 COOH 



By these methods, and modifications of these methods, Fischer and 

 others have succeeded in building up long chains of amino-acid-groups, 

 these chains being collectively termed by Fischer, Peptides. Chains 

 consisting of two links, i. e., combinations of two amino-acids are 

 designated Dipeptides; such, for example, are glycyl-glycine, alanyl- 

 alanine and leucyl-leucine, chains consisting of three links are termed 

 Tripeptides, such being, for example, diglycyl-glycine and leucyl-glycyl- 

 glycine. Chains consisting of four links are termed Tetrapeptides, 

 and so on, the higher members of the series being collectively termed 

 Polypeptides. 



The surpassing interest of these investigations lies in the fact that 

 many of the polypeptides are considered to be, in all probability, 

 identical with certain of the natural peptones derived from proteins by 

 partial hydrolysis, while others probably merit inclusion among the 

 proteins themselves. Thus the Octadecapeptide, 1-leucyl-triglycyl- 

 1-leucyl-triglycyl-l-leucyl-octaglycyl-glycine, and the Tetradecapeptide, 

 1-leucyl-triglycyl-l-leucyl-octaglycyl-glycine so closely resemble, in 

 general properties, the ordinary proteins, that they would undoubtedly 

 have been classed among the proteins had they been first met with in 

 nature. Thus, they give the biuret-reaction, and form opalescent 

 watery solutions, and the tetradecapeptid is coagulated by ammonium 

 sulphate and precipitated by tannic acid and by phosphotungstic acid. 

 As they do not contain tyrosine, tryptophane, phenylalanine or 

 cystine, they fail to give such protein color reactions as depend upon 

 the presence of these groups. 



The molecular weight of the octadecapeptide is 1213, and the sub- 

 stitution of phenylalanine, tyrosine and cystine in the place of glycine 

 groups would increase this weight two or three times, giving a value 

 which is of the same order of magnitude as the more modern esti- 

 mations of the (minimal) molecular or combining-weights of many of 

 the natural proteins. 



A whole series of the polypeptides give the typical peptone biuret 

 reaction (pink), and such as contain tyrosine also give Millon's reac- 

 tion. The biuret-reaction is, with the glycine compounds, first encoun- 

 tered in the tetrapeptide, but it is given by tripeptides built up to 

 include other amino-acids. The biuret-reaction is, generally speaking, 

 more intense the greater the length of the polypeptide chain. 



The majority of the polypeptides are readily soluble in water, and 

 such as are soluble in water with difficulty, are readily soluble in dilute 

 mineral acids and alkalies, with which they combine; they are less 

 soluble in solutions of acetic acid. As a rule they are insoluble in 

 absolute alcohol, but in alcohol containing a little watery ammonia 



