56 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



This quotient in the case of the monoamino acids was I ; in the 

 case of arginine it was { ; in that of histidine . 



There is thus a distinct regularity in the fixation of carbon dioxide ; 

 the amino groups of aliphatic amino acids are quantitatively converted 

 into carbamino groups : in histidine, arginine, tryptophane only the 

 amino group of the side chain, not the nitrogen atoms of the rings, 

 reacts forming a carbamino group. 



Amino groups in benzene nuclei do not react except under certain 

 conditions : p-aminophenol reacts quantitatively ; p-phenylene diamine 

 does not react quantitatively (Sulze). 



Primary amines react quantitatively : tertiary amines do not 

 react 



Not only do amino compounds react, but also alcohols, carbo- 

 hydrates and hydroxy acids. Carbon disulphide forms similar com- 

 pounds with amino acids. 



Glycyl-glycine was found by Siegfried to react with carbonic acid 

 in the presence of barium hydrate with the formation of the barium 

 salt of glycyl-glycine carbamino acid, which on heating was converted 

 into barium carbonate and glycyl-glycine. Siegfried and Liebermann 

 have found that the peptide linking in the polypeptides reacts to a 

 certain extent. Dipeptides give a quotient of T 4^ to ^-4?, tri pep- 

 tides of ^ryy and tetrapeptides of ^ ff , instead of I, J and \. 



Peptones and the proteins of serum as well as the amino acids react 

 with carbonic acid in the presence of calcium salts ; this may explain 

 certain of the phenomena concerning the presence of carbonic acid in 

 blood and in working muscle ; a protein carbonic acid compound may be 

 formed which can give rise to carbonic acid without taking up oxygen. 



The reaction may be made use of for separating glycine and alanine ; 

 the calcium salt of glycine carbamino acid is almost insoluble, but the 

 alanine carbamino salt is comparatively soluble. It has been found of 

 use by Siegfried for proving that his intermediate compounds the 

 kyrines in the hydrolysis of proteins are not mixtures of amino acids, 

 and for showing that certain peptones can be isolated from solution 

 and their physical properties examined, and proving whether they are 

 still so complex a mixture. 



Siegfried's results with glycine and glycyl-glycine have been con- 

 firmed by Leuchs, who, in addition, has investigated the combinations 

 of amino acids and of polypeptides with carbonic acid which were pre- 

 pared by Fischer and his pupils by combination with chlorocarbonic 

 ester. 



