40 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



The Properties of the Polypeptides. 



The physical properties of the various polypeptides show generally 

 much resemblance to one another, although many differences have been 

 observed. 



The majority are easily soluble in water ; the exceptions amongst 

 the dipeptides are, e.g., dl-leucyl-glycine, leucyl-alanine, and leucyl- 

 leucine; also phenylalanyl-glycine, phenylalanyl-phenylalanine, and 

 some others ; amongst the tripeptides, leucyl-alanyl-alanine A, phenyl- 

 alanyl-glycyl-glycine, leucyl-glycyl-phenylalanine ; amongst the tetra- 

 peptides, dileucyl-glycyl-glycine. In contradistinction to the other 

 polypeptides made up entirely of glycine units, the pentapeptide and 

 the hexapeptide are soluble with difficulty even in hot water. 



Of the complex polypeptides, the octapeptide, 1-leucyl-hexaglycyl- 

 glycine, is the most soluble in water, and the decapeptide, 1-leucyl- 

 octaglycyl-glycine, the least soluble ; the solubility increases again in 

 the case of the tetradeca- and octadecapeptides ; their warm clear 

 aqueous solutions become opalescent on cooling. 



In general, the solubility in water of mixed polypeptides is greater 

 than the solubility of the polypeptides made up of a single amino 

 acid; the ready solubility of the dipeptides glycyl-1-tyrosine, leucyl- 

 tyrosine, which contain the amino acids soluble with difficulty in water, 

 should also be noted. 



Most of the polypeptides are insoluble in alcohol ; leucyl-proline is, 

 however, an exception, for it dissolves both in alcohol and in acetic 

 ester somewhat easily. 



Those polypeptides which are soluble with difficulty in water, are 

 dissolved easily by mineral acids and alkalies with the formation of 

 salts ; they are less soluble in acetic acid. In many cases they may be 

 dissolved in alcohol if a few drops of ammonia be added ; they separate 

 out on boiling off the ammonia. 



Certain polypeptides, for instance, leucyl-diglycyl-glycine, in the 

 amorphous state are soluble in alcohol, but they are changed on warming 

 into their insoluble crystalline form. 



Most of the polypeptides melt above 200 C. and at the same time 

 undergo decomposition. The dipeptides when fused are converted 

 into their diketopiperazines. Certain of the glycine polypeptides are 

 decomposed without melting. Leucyl-proline again, as in many other 

 of its properties, is an exception, as it melts at 116-119 C. 



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

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



