CYSTINE. 149 



he has also established the relation between cystine and taurinc. Cys- 

 tine is the disulphide of cysteine, which is a-amino-/3-thiolactic acid. 

 From cysteine bv oxidation FRIEDMANN obtained cysteinic acid, 



CH 2 SO 2 OH 

 C3H 7 NS05 = CH(NK 2 ), from which taurine CH 2 (S0 2 OH) is produced'by 



COOH CH 2 (NH 2 ) 



splitting off C0 2 . 



Cystine has also been prepared synthetically in several ways. For 

 example, FISCHER and RASKE 1 have prepared cystine from a-amino 

 /3-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 (O:)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 cysteine. 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 S ) melts at 180-181; the phenylisocyanate 

 compound at 160. On boiling w T ith 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. 



