POLYPEPTIDES 51 



The Properties of the Polypeptides. 



/. Solubility, Optical Properties, etc. 



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, dl-leucyl-glycine, leucyl-alanine and leucyl-leucine ; 

 also phenylalanyl - glycine, phenylalanyl - pheny lalanine and some 

 others ; amongst the tripeptides, leucyl-alany 1-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 polypeptides containing 

 different units is greater than the solubility of the polypeptides made 

 up of a single amino acid ; the ready solubility of the dipeptides 

 glycyl-1-tyrosine and leucyl-tyrosine which contains the amino acids 

 soluble with difficulty in water, should be noted. 



Most of the polypeptides are insoluble in alcohol. Certain poly- 

 peptides, for instance leucyl-diglycyl-glycine, in the amorphous state 

 are soluble in alcohol, but they are changed on warming into their 

 insoluble crystalline form. 



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. 



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

 dodeca- and the tetradeca-peptides give salts with mineral acids which 

 are soluble with difficulty ; only the lower ones give soluble salts. 



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

 undergo decomposition. The dipeptides are converted into their 

 diketopiperazines when they are fused. Certain of the glycine poly- 

 peptides are decomposed without melting. 



