88 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



Leucine. 



A substance, corresponding to our leucine, was described by Proust 

 in 1818 under the name of oxide-caseux. Two years later, in 1820, 

 Braconnot isolated from the products resulting by boiling meat with 

 dilute sulphuric acid a substance which he named leucine on account 

 of its glistening, white (Xeu/co?) appearance. Mulder, in 1839, obtained 

 it by boiling meat with alkali and by the putrefaction of casein. Its 

 occurrence and oxidation products were investigated by Liebig, who 

 regarded it as one of the constituents of the protein molecule, as was 

 proved in 1 849 by Bopp, who prepared it from caseinogen, fibrin and 

 albumin by fusion with potash, and also by hydrolysis with acids and 

 by putrefaction. Hinterberger showed that it was present in horn, 

 and Zollikofer in elastin. 



Leucine also occurs in the free state in the various organs of the 

 animal body as has been pointed out by Frerichs and Stadeler and 

 many other observers. 



Not only is it present in animal, but also in vegetable, proteins, 

 from which it passes by the action of enzymes into the extracts of 

 germinating seedlings, as shown by Schulze and his co-workers. 

 Leucine is, with the exception of arginine, the most widespread of 

 all the units which go to make up the protein molecule. 



Its correct empirical formula C 6 H 13 NO 2 was first given to it by 

 Laurent and Gerhardt. These observers and also Cahours showed that 

 it belonged to the glycine series of compounds ; Liebig and others 

 showed that on oxidation it gave ammonia and valerianic acid, and 

 also valeronitrile, and Strecker obtained leucic acid by treating it with 

 nitrous acid. But only in 1868, when Hiifner obtained caproic acid 

 and ammonia by reducing it with hydriodic acid, was it shown to be 

 an a-aminocaproic acid. Hiifner tried to prove this by comparing the 

 natural leucine with two synthetical leucines ; (i) that prepared by the 

 action of ammonia on bromocaproic acid obtained from the fermenta- 

 tion caproic acid, and (2) that prepared from isovaleraldehyde, hydrogen 

 cyanide and ammonia which had been first synthesised by Limpricht in 

 1855. Neither of these two synthetical leucines corresponded exactly 

 with natural leucine, and Hiifner regarded them as identical com- 

 pounds rather than as isomers. 



The question of the constitution of leucine was again taken up in 

 1891 by Schulze and Likiernik. The natural product is optically 

 active, but on heating with baryta at 160 C. it is racemised; this 



