PROTEINS 



79 



of pure leucine are reproduced in Fig. 28. It is rather easily soluble in 

 water (46 parts), alkalis, ammonia, and acids. On rapid heating to 

 295C., leucine decomposes with the formation of carbon dioxide, ammo- 

 nia, and amylamine. Aqueous solutions of leucine obtained from pro- 

 teins are levo-rotatory, but its acid or alkaline solutions are dextro- 

 rotatory. So-called impure leucine 1 is a slightly refractive substance, 

 which generally crystallizes in balls having a radial structure, or in 

 aggregations of spherical bodies, Fig. 145, Chapter XXV. 



Isoleucine, C 6 Hi 3 NO 2 . Isoleucine is a-amino-0-methyl-p-ethyl-pro- 

 pionic acid, and possesses the following structural formula: 



CH 3 NH 2 



I I 



H C - C COOH. 



I I 



C 2 H 6 H 



FIG. 28. LEUCINE. 



This amino acid was discovered by Ehrlich in 1903. ' Its presence has 

 been established among the decomposition products of only a few pro- 

 teins, although it probably occurs among those of many or most of them. 

 Ehrlich has shown that the d-amyl alcohol which is produced by yeast 

 fermentation originates from isoleucine and the isoamylalcohol origi- 

 nates from leucine. Isoleucine is dextro-rotatory. 



Lysine, CH 2 (NH 2 ) CH 2 -CH 2 -CH 2 -CH(NH 2 ) -COOH. The three 

 bodies, lysine, arginine, and histidine, are frequently classed together 

 as the hexone bases. Lysine was the first of the bases discovered. It 

 is a--diamino-caproic acid and hence possesses the following structure. 



1 These balls of so-called impure leucine do contain considerable leucine, but inasmuch 

 as they may contain many other things it is a bad practice to allude to them as leucine. 



