148 THE PROTEIN SUBSTANCES. 



caustic alkali it decomposes and yields alkali sulphide, which can be 

 detected by lead acetate or sodium nitroprusside. According to MOR- 

 NER 1 75 per cent of the total sulphur is separated. On treatment of 

 cystine with tin and hydrochloric acid it develops only a little sulphu- 

 retted hydrogen, and is converted into cysteine. 



On heating upon platinum-foil cystine does not melt, but 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 evaporation 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 precipitation 

 with mercuric acetate is especially suited. Thebenzoyl cystine (BAUMANN 

 and GoLDMANN 2 ) melts at 180-181; the phenylisocyanate com- 

 pound at 160. On boiling with 25 per cent hydrochloric acid this com- 

 pound passes to the anhydride, which is a hydantoin melting at 119 C. 



Stone-cystine, according to NEUBBRG and MAYER, differs in many respects 

 from the ordinary cystine, among which the following will be mentioned: The 

 optically active stone-cystine crystallizes in needles, the specific rotation is 

 ()D=-206; it melts at 190-192 with marked swelling up. The benzoyl 

 compound melts at 157-159; the phenylcyanate 'compound melts at 170-172, 

 and it is not changed on boiling with hydrochloric acid. 



In the detection and identification of cystine we make use of the 

 crystalline form, the behavior on heating on platinum-foil, and the sulphur 

 reaction after boiling with alkali. As to its preparation from protein 

 substances see K. MORNER. S In regard to the detection of cystine in the 

 urine see Chapter XV. 



CH 2 .SH 

 Cysteine (a-amino-/?-thiolactic acid), C 3 H 7 NSO2 = CH(NH2), is formed from 



COOH 



cystine by reduction with tin and hydrochloric acid. It is also produced in the 

 cleavage of protein substances, but this is considered by MORNER and PATTEN 4 

 as a secondary formation. Cysteine can be easily converted into cystine by 

 oxidation. 



Toward alkalies and lead acetate it acts like cystine. With sodium nitro- 

 prusside and alkali it gives a deep purple-red coloration; with ferric chloride the 

 solution gives an indigo-blue coloration which quickly disappears. 



CH 3 



Thiolactic acid (a-thiolactic acid) C 3 H 6 SO 2 = CH(SH), has been found once 



COOH 



as a cleavage product of ox-horn by BAUMANN and SUTER. MORNER, 

 FRIEDMANN and BAER obtained it from cystine. It has been shown by FRIED- 



1 Zeitschr. f. physiol. Chem., 34. 3 Zeitschr. f . physiol. Chem., 34. 



2 Ibid., 12. 4 See footnote 1, page 79. 



