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124 THE PROTEIN SUBSTANCES.
is identical with 3-5 di-iodo-tyrosine, HOI2 C6H2.CH2 .CHNH2COOH,
synthetically prepared by Wheeler and Jamieson.1
On acid cleavage
of gorgonin, Henze 2
obtained the three hexone bases, abundant tyrosine
and ’very little leucine. On cleavage with barium hydroxide he obtained
only lysine, besides tyrosine and glycocoll in larger amounts.
Fibroin and sericin are the two chief constituents of raw silk. By
the action of boiling water the sericin (silk gelatin) dissolves and can be
obtained by a method suggested by Bondi,3
while the more difficultly
soluble fibroin remains undissolved in the shape of the original fiber.
The sericin, whose sufficiently concentrated hot solution gelatinizes on
cooling, is precipitated by mineral acids, several metallic salts, and by
acetic acid and potassium ferrocyanide. The spider silk investigated
by Fischer 4
yielded fibroin but not sericin.
Abderhalden and his collaborators 5
have investigated a great
number of varieties of silk and found sericin in varying amounts (15 to
28 per cent). The composition of the various kinds of silk is char-
acterized, especially, by a varying amount of glycocoll and in this regard
we can differentiate between two chief groups. The one group is, like
the Italian silk, very rich in glycocoll while the other group, like the
Tussah silk, contains a much smaller quantity of glycocoll.
Sericin, whose proper concentrated warm solution gelatinizes on
cooling, is precipitated by mineral acids and several metallic salts and
by acetic acid and potassium ferrocyanide. In regard to the products
of hydrolysis it differs very essentially from fibroin by being much poorer
in glycocoll, alanine and tyrosine.
Fibroin is soluble in concentrated acids and alkalies and reprecipitable
(in a modified form) on neutralization. It gives the biuret test and
Millon’s and Adamkiewicz-Hopkiih’s reactions, the last but faintly.
Fibroin has an especially great interest because of the hydrolyses per-
formed by Fischer and his co-workers, and especially by the finding of
the previously mentioned polypeptides by these workers. Of the cleavage
products which characterize fibroin we must mention the large amount
of glycocoll, alanine and tyrosine, and the very small amounts of hexone
bases, besides the almost complete absence of monamino-dicarboxylic
acids. The quantity of the hydrolytic cleavage products of the three
silk substances, in so far as they have been investigated, are given in the
following table, which also includes the results for elastin, gelatin, and
1
Wheeler and Jamieson, Amer. Chem. Journ., £3; Wheeler, ibid., 38.
2
Henze, Zeitechr. f. physio). Chem., 38 and 51.
’ Zeitschr. f. physiol Chem., 34.
* Ibid., 58.
8
See Zeitschr. f. physiol. Chem., 59, 61, 62, 64, 71, 74, 80.
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