THE SYNTHESIS OF THE PROTEINS 41 



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 in their bitter taste to 

 the natural peptones 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. Multirotation 

 has not yet 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 diketo- 

 piperazines, whose solutions remain colourless, i.e., they do not give 

 copper salts. Leucyl-proline forms again an exception. 



The high molecular polypeptides, such as the octa-, the deca- up to 

 the tetrad eca-peptides, give salts with mineral acids which are soluble 

 with difficulty, but the lower ones give soluble salts as before men- 

 tioned. 



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 

 expected 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. 



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 intenser as the length of the polypeptide 

 chain increases, and the colour is also more intense when the carboxyl 

 group is esterified ; this is especially noticeable in the case of triglycyl- 

 glycine and its ester, the so-called biuret base. The same occurs when 



