1 1 2 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



B. MONOAMINODICARBOXYLIC ACIDS. 



Aspartic Acid. 



Asparagine, the amide of aspartic acid, was first isolated by Robi- 

 quet and Vauquelin, in 1 806, from the juice of Asparagus officinalis ; 

 hence its name. Not only is asparagine found in asparagus, but also 

 in the seedlings of lupines, peas, vetches, etc., from which it is best and 

 most easily prepared. The natural compound is 1-asparagine. The 

 isolation of d-asparagine by Piutti has been proved by Pringsheim to 

 be due to racemisation of the natural 1-aspargine in the process of 

 extraction, and the fact that the two isomers can be separated by 

 fractional crystallisation. 



Aspartic acid was first obtained by Plisson, in 1827, from asparagine 

 by boiling it with lead hydroxide, and is usually prepared from this 

 compound by hydrolysis with alkali, or acid. 



Only however in 1868 was the presence of aspartic acid in vegetable 

 proteins shown by Ritthausen, who obtained it by the hydrolysis of 

 conglutin and legumin with sulphuric acid ; in the following year 

 Kreussler obtained it in the same way from animal proteins. In 1 874 

 Radziejewski and Salkowski found that it was a product of the tryptic 

 digestion of proteins, and the asparagine in plants most probably arises 

 from the aspartic acid of the protein in the seed. 



Its composition, C 4 H 7 NO 4 , was established in 1833 by Boutron- 

 Charlard and Pelouze, and confirmed by Liebig. In 1848 Piria showed 

 that aspartic acid was converted into malic acid by the action of nitrous 

 acid, and he regarded aspartic acid and asparagine as the two amides 

 of malic acid 



CONH 2 . CH 2 . CHOH . COOH CONH 2 . CH 2 . CHOH . CONH 2 



corresponding to oxamic acid and oxamide. 



This idea of their constitution was proved to be erroneous by Kolbe 

 in 1862, who showed that aspartic acid did not give off ammonia when 

 boiled with dilute caustic alkali, and that asparagine only lost half of 

 its nitrogen when thus treated. Aspartic acid was therefore not the 

 amide of malic acid, but amino-succinic acid, and asparagine was the 

 amide of this compound. 



The first synthesis of aspartic acid is that by Dessaignes in 1850, 

 who obtained a crystalline substance by heating acid ammonium malate 

 to 160-200 C., which, when treated with hydrochloric or nitric acid, 

 was converted into aspartic acid. In the same way Dessaignes ob- 



