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CYSTINE. 149
he has also established the relation between cystine and taurine. Cys-
tine is the disulphide of cysteine, which is a-amino-/3-thiola<t ic acid.
From cysteine by oxidation Friedmann obtained cysteinic acid,
CH2SO2OH
C3H7NS06 =CH(NH2), from which taurine CH2 (S02 OH) is produced’by
COOH CH2 (NH2 )
splitting off C02 .
Cystine has also been prepared synthetically in several ways. For
example, Fischer and Raske * have prepared cystine from a-amino-
/S-chlorpropionic acid (obtained from Z-serine) by the action of barium
hydrosulphide and a subsequent oxidation in the air.
/-Cystine crystallizes in thin, colorless, hexagonal plates. It is not
soluble in water, alcohol, ether, or acetic acid, but dissolves in mineral
acids and oxalic acid. It is also soluble in alkalies and ammonia, but
not in ammonium carbonate. Cystine is optically active, being levorota-
tory. Morner found it to be (a) D = —224.3°. On heating with hydro-
chloric acid it can, according to Morner, be changed into a modifica-
tion crystallizing in needles and with a weaker levorotatory power, or
indeed dextrorotatory, composed of a mixture of the two optically active
cystines. On heating with HC1 to 165° for 12-15 hours Neuberg and
Mayer obtained inactive cystine. By fungus fermentation with Asper-
gillus niger they obtained dextrorotatory cystine. Cystine has no
melting-point but slowly decomposes at 258-261°. On boiling cystine
with caustic alkali it decomposes and yields alkali sulphide, which can
be detected by lead acetate or sodium nitroprusside. According to Mor-
ner 2
75 per cent of the total sulphur is separated. Cystine treated
with tin and hydrochloric acid develops only a little sulphureted
hydrogen, and is converted into cj^steine. Cystine yields sulphureted
hydrogen and methyl mercaptan on putrefaction.
On heating upon platinum-foil cystine ignites and burns with a bluish-
green flame, with the generation of a peculiar sharp odor. When warmed
with nitric acid it dissolves with decomposition, and leaves on evapora-
tion a reddish-brown residue, which does not give the murexid test.
Cystine is gradually precipitated from its sulphuric acid solution by
phosphotungstic acid. Cystine forms crystalline salts with mineral
acids and with bases. For isolating and separating cystine the precipita-
tion with mercuric acetate is especially suited. The benzoyl cystine
(Baumann and Goldmann 3
) melts at 180-181°; the phenylisocyanate
compound at 160°. On boiling with 25 per cent hydrochloric acid this
1
See Erlenmeyer and Stoop, Ber. d. d. chem. Gesellsch., 36; Gabriel, ibid., 38; Fischer
and Raske, ibid., 41.
2
Zeitschr. f. physiol. Chem., 34.
3
Ibid., 12.
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