ALBUMINS OR PROTEINS. 151 



The two varieties can be easily recognized by their crystalline structure. 

 One is left-handed hemihedral, and the other right-handed. The d-variety 

 tastes sweet, while the other is insipid. 



Glutamine, the amide of amino-glutaric acid, has been found in germi- 

 nating pumpkin seeds, by Schulze and Barbieri. 1 Its occurrence is simi- 

 lar to that of asparagine. 



As far as we know, these two amides do not occur in the animal organism. 

 We shall later on consider their value as a food material. 



Before discussing the properties of the di-amino acids, we shall devote 

 a little attention to the remaining mono-amino acids. Phenylalanine, first 

 discovered by Schulze and Barbieri 2 in the embryo of lupines, has recently 

 been recognized as an invariable constituent of all the albumins so far 

 investigated. Its constitution is that of phenyl-amino-propionic acid: 



C 6 H 5 . CH . CHNH 2 COOH. It occurs in nature as the /-variety. 



x- 



Another aromatic amino acid, which has been known for a long time 

 as a constituent of albumin, is tyrosine, which can be easily isolated on 

 account of its difficult solubility in water. It is p-hydroxy-phenyl- 

 amino-propionic acid: C 6 H 4 OH . CH 2 . CH(NH 2 ) . COOH. It occurs in 



nature in both modifications, although mainly in the laevo form. 



Tyrosine gives several color reactions, of which that of Hoffmann is the 

 most important. It is known under the name of Milton's reaction. If 

 tyrosine is boiled with a nitric acid solution of mercuric oxide, containing 

 a little nitrous acid, the liquid becomes colored, and the resulting precipi- 

 tate is rose-colored or a dark brownish red. This reaction is not confined 

 to tyrosine. It is given by all benzene derivatives, in which a hydrogen 

 atom has been substituted by a hydroxyl group. This reaction has 

 become of great importance on account of the fact that all proteins 

 which contain tyrosine will give it. Millon's reaction is, therefore, a test 

 for proteins. 



We now come to the heterocyclic compounds. Two representatives of 

 this class have been discovered by Emil Fischer: 8 a-pyrrolidine-carboxylic 

 acid, also called proline, and hydroxy-pyrrolidine-carboxylic acid; the latter 

 being probably an hydroxy-a-pyrrolidine-carboxylic acid: 



CH 2 CH2 



and C 5 H 9 N0 3 . 

 % CH 2 *CH . COOH 

 \ / 

 NH 

 Proline Hydroxy-proline 



1 E. Schulze and J. Barbieri: Ber. 10, 199 (1877). 

 * Ibid. 14, 1785 (1881); Z. f. physiol. Chem. 12, 405 (1888). 



3 E. Fischer: Z. physiol. Chem. 33, 151 (1901); Ber. 36, 2660 (1902). Cf. H. Leuchs: 

 Ibid. 38, 1937 (1905). 



