52 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



The taste of the polypeptides is not sweet, like that of the amino 

 acids, but slightly bitter ; some of the isomeric polypeptides possess 

 distinct differences in their taste ; thus leucyl-alanine is tasteless, but 

 the two alanyl-leucines have a bitter taste. The presence of a-amino 

 acids amongst polypeptides may even be recognised by their sweet 

 taste, and the resemblance of the polypeptides to the natural peptones 

 in their bitter taste is very remarkable. 



The optically active polypeptides have generally a very high 

 specific rotation in comparison with the amino acids ; but the rotation 

 is very changeable just as in other classes of compounds. Mutarotation 

 has not been observed. This property has proved very valuable 

 in the study of the hydrolysis of the polypeptides by the action of 

 enzymes. 



The chemical properties of the polypeptides depend greatly on 

 their complexity. Like amino acids, all the ordinary polypeptides, 

 when their solutions are boiled with precipitated copper oxide, give 

 blue, sometimes blue-violet solutions, and in this way differ from the 

 diketopiperazines, whose solutions remain colourless, i.e., they do not 

 give copper salts. Kober is now studying these copper salts. 



II. Reactions. 



The simple polypeptides, like the a-amino acids, give no precipitate 

 with phosphotungstic acid, but this condition depends on the length of 

 the polypeptide chain. Many tripeptides, such as leucyl-glycyl-glycine, 

 give a precipitate with phosphotungstic acid in the presence of sulphuric 

 acid if their solution be not too dilute, and this occurs with almost all 

 the tetrapeptides. The derivatives of the diamino acids behave as ex- 

 pected in giving a precipitate with phosphotungstic acid. 



The octa-, deca-, etc., peptides are immediately precipitated by 

 phosphotungstic acid ; they are also thrown down by tannic acid and 

 by concentrated ammonium sulphate solutions. They resemble, in fact, 

 many natural proteins and would have been regarded as such if they 

 had been found in nature. They lack only the colour reactions due to 

 the absence of tyrosine, tryptophane, etc., in their molecules. 



Di-1-leucyl-cystine, leucyl-triglycyl-tyrosine, and other tripeptides 

 containing tyrosine, resemble the proteoses very closely in their be- 

 haviour with ammonium sulphate. 



The biuret reaction is positive with the greater number of the 

 polypeptides excluding the dipeptides. In the case of the glycine 

 compounds it occurs first with the tetrapeptide, but it occurs with other 

 tripeptides. It is distinctly more intense as the length of the poly- 



