THE AROMATIC OXY-ACIDS. 289 



CH 2 .SH 

 2CH.NH 2 



COO] 



)H 



Cyste'in. Cystin. 



On oxidation with bromine it is possible to transform the loosely 

 combined sulphur into oxidized sulphur with the production of 

 cystei'nic acid, which apparently represents the sulpho-acid of cy stein. 

 Its probable formula is 



CH 2 .S0 2 OH 



CH.NH 2 

 COOH 



Through loss of CO 2 this then gives rise to taurin (which see). 



Two varieties of cystin exist, of which one is Isevorotatory and 

 the other dextrorotatory. The variety which is found in the urine 

 is Ia3vorotatory. The substance usually crystallizes in colorless 

 hexagonal platelets which are very characteristic in appearance. 

 But it may also separate out in needles, which differ from those of 

 tyrosin in the fact that they appear more highly refractive under 

 the microscope and present obliquely cut ends. On recrystallization 

 from a 10 per cent, solution of ammonia these needle-like crystals 

 disappear and are replaced by hexagonal platelets. 



Cystin is soluble in solutions of the alkaline hydrates, in ammonia, 

 and the mineral acids. In water, alcohol, ether, and acetic acid it 

 is insoluble, as also in solutions of ammonium carbonate ; for this 

 reason the cystin is apt to crystallize out from decomposing urines, 

 when previously it has been present in solution only. 



On heating cystin on platinum foil it does not melt, but ignites 

 and burns with a bluish-green flame ; at the same time a peculiar, 

 penetrating odor develops. It does not give the murexid reaction. 

 When boiled with caustic alkali it is decomposed and the sulphur 

 liberated as a sulphide. With benzoyl chloride, in the presence of 

 an excess of caustic alkali, it forms benzoyl-cystin, and is thus pre- 

 cipitated as a sodium salt in the form of fine lustrous platelets, 

 which are readily soluble in water, but insoluble in solutions of the 

 caustic alkalies. Upon the addition of an acid to such a solution, 

 benzoyl-cystin separates out as such. It is soluble in alcohol and 

 alcohol-ether, slightly so in pure ether, and almost insoluble in 

 water. Its needle-like crystals melt at 156-158 C. The forma- 

 tion of benzoyl-cystin may be expressed by the equation : 



CH 2 .S S.CH 2 CH 2 .S S.CH 2 



2C 6 H 5 .COC1+CH.NH 2 CH.NH 2 =CH.NH.C 6 H 5 CO C 6 H 5 .CO.NH.CH + 2HC1 



COOH COOH COOH COOH 



Cystin. Benzoyl cystin. 



