16 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



Combinations of Amino Acids with Carbonic Acid. 



The sodium and barium salts of the monoamino acids have a strongly 

 alkaline reaction and are highly dissociated salts. If carbonic acid be 

 passed into the solution of the barium salt, barium carbonate is not, 

 as would be expected, immediately formed ; the solution remains clear, 

 and only after a short time, when the solution becomes saturated with 

 carbonic acid, does it become cloudy and barium carbonate gradually 

 separates out ; the separation of barium carbonate is hastened by heat- 

 ing. This phenomenon is due, as was shown by Siegfried in 1905, to 

 the formation of salts ofcarbamino acids of the general formula 



xH 



R N/ 



COOH 



i.e., to the formation of a dibasic acid of which the calcium salt, 



/H 



R N< 



L\COO 

 / 

 Ca 



is soluble with difficulty in ice-cold water and alcohol. Similar com- 

 pounds are formed with the dibasic aspartic and glutamic acids and 

 with the diamino acids. In aqueous solutions also the free carbamino 

 acid is formed. The reaction may serve, as Siegfried pointed out in 

 1906, for the separation of amino acids from their solutions. 



Siegfried and Neumann, in 1908, showed that there was a distinct 

 regularity in the fixation of carbonic acid by amino" acids ; the amino 

 groups of the aliphatic amino acids were quantitatively converted into 

 carbamino groups ; in histidine and arginine only the amino group of 

 the side chain, not the nitrogen atoms of the rings, reacted with carbonic 

 acid to form carbamino groups. 



Glycyl-glycine was also 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. Further, Siegfried and 

 Liebermann have shown that the peptide linking in the polypeptides 

 reacts to a certain extent, and by this means they hope to obtain an 

 idea of the constitution of the various peptones which Siegfried has 

 isolated from proteins by the action of trypsin. 



Not only do the amino acids react with carbonic acid in the presence 

 of calcium salts, but also peptones and the proteins of serum ; this may 



