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144 THE PROTEiN SUBSTANCES.
is an isomer of leucine discovered by F. Ehrlich, 1
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
3’ields 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
f (aj 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,
C6H13NO2 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, Her. d. d. chem. Gesellsch., 37.
2
Journ. of Biol. Chem., 8.
1
Ehrlich, Ber. d. d. chem. Gesellsch., 40 and 41; Brasch and Friedmann, Hof-
meister’e Beitrage, 11; Bouveault and Locquin, Compt. rend., 141, and Bull. soc.
fhim. (3), :{•">: Locquin, Bull. boc. chim. (4), 1.
;
C. Neuberg, Bioch. Zeitachr., 37.
*Zeitschr. f. physiol. Chem., 81 and 84.
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