144 THE PROTEIN SUBSTANCES. 



is an isomer of leucine discovered by F. EHRLICH/ who first isolated 

 it from the mother-liquor after removing the sugar from beet-sugar 

 molasses. He also found it in the hydrolysis of several proteins, and 

 recently it has been found by others among the products of hydrolysis 

 of the proteins. The largest amount thus far found was 2.6 per cent 

 by LEVENE, v. SLYKE and BIRCHARD 2 in a heteroproteose. It seems 

 to be associated regularly with ordinary leucine, forming mixed crystals, 

 which give an impression of a chemical combination and which are dif- 

 ficult to separate. On this account the earlier claims as to the quantity 

 of leucine are somewhat uncertain, as they always refer to leucine 

 containing isoleucine. 



The constitution of isoleucine has been explained by EHRLICH through 

 its relation to d-amyl alcohol. Just as according to F. EHRLICH valine 

 yields the isobutyl alcohol in alcoholic fermentation so isoleucine yields 

 d-amyl alcohol in the fermentation of sugar with yeast. On the other 

 hand, it can also be obtained, in a manner analogous to the synthesis 

 of leucine, from d-amyl alcohol (as a mixture of isoleucine and alloiso- 

 leucine, the latter is levogyrate and has a different stereometric configura- 

 tion from the isoleucine). The synthesis of isoleucine has been accom- 

 plished in other ways by several investigators. 3 On putrefaction d-ca- 

 proic acid and d-valeric acid have been obtained from isoleucine. 4 



Isoleucine crystallizes in leaves or rods and plates of the rhombic 

 form. It is more soluble in water than leucine (1:25.8). Its solutions 

 have a bitter taste and are astringent. It is dextro-rotatory in aqueous 

 as well as in acid solution. In aqueous solution it has a specific rotation 

 of (a) D = -}-9.74 and in 20 per cent hydrochloric acid (a) D = +36.8. 

 Like valine its copper salt is readily soluble in methyl alcohol. The 

 benzoyl combination melts at 116-117, the benzene sulphoisoleucine 

 at 149-150, the phenylisocyanate combination at 119-120, and the 

 naphthylisocyanate combination at 178 C. 



In the leucine fraction, from the amino-acids contained in nerve 

 substance, ABDERHALDEN and WEIL 5 have obtained a new amino-acid, 

 C6HisN02 which is isomeric with leucine and which seems to be d-a- 

 amino-n-caproic acid and called d-caprine by them. When crystallized 

 from water it forms six-sided plates which unite to tufts having a faint 

 sweet taste. At 280 (uncorrected) it softens and at 285 (uncorrected) 



1 Felix Ehrlich, Ber. d. d. chem. Gesellsch., 37. 



2 Journ. of Biol. Chem., 8. 



'Ehrlich, Ber. d. d. chem. Gesellsch., 40 and 41; Brasch and Friedmann, Hof- 

 meister's Beitrage, 11; Bouveault and Locquin, Compt. rend., 141, and Bull. soc. 

 chim. (3), 35; Locquin, Bull. soc. chim. (4), 1. 



4 C. Neuberg, Bioch. Zeitschr., 37. 



6 Zeitschr. f . physiol. Chem., 81 and 84. 



