POLYPEPTIDES 55 



Carbonic Acid Derivatives of Amino Acids and Polypeptides. 



The sodium, calcium and barium salts of the monoamino acids 

 have a strongly alkaline reaction and are highly dissociated in solution. 

 If carbonic acid be passed into the solution of the barium salt, barium car- 

 bonate 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 due to the gradual 

 separation of barium carbonate ; the precipitation of barium carbon- 

 ate is hastened by heating. This phenomenon is due, as was shown by 

 Siegfried in 1905, to the formation of salts of carbamino acids of the 

 general formula, 



/ H 



R N/ 



COOH C H 



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



/ H 



R N< 



\COO 



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 the free carbamino acid 

 is formed. 



The combination of amino acids and other nitrogenous compounds 

 with carbon dioxide is readily ascertained, as was first shown by Sieg- 

 . fried and Neumann in 1908. Carbon dioxide is passed into the solu- 

 tion of the amino compound in lime water, the solution is filtered and 

 boiled. Calcium carbonate separates out. By weighing the calcium 

 carbonate and estimating the nitrogen in the solution the ratio of the 

 two can be determined. If these figures be divided by the molecular 

 weight of calcium carbonate and the atomic weight of nitrogen, the 

 resulting figure gives the number of molecules of carbon dioxide in 

 combination with the number of nitrogen atoms ; by reducing the 

 carbon dioxide figure to unity the number of reacting nitrogen atoms 

 is obtained from the quotient 



CO, i 

 TT"i 



where x is the number of nitrogen atoms taking up one molecule of 

 carbon dioxide. 



