144 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



in solutions of caustic alkali and alkali carbonates. It crystallises from 

 its solution in ammonia in the typical hexagonal plates when the 

 solution is allowed to evaporate. This is the most convenient way 

 to obtain crystals for microscopic examination and identification by 

 crystalline form. 



If cystine be heated on platinum foil it burns with a bluish-green 

 flame without melting. 



The presence of sulphur in the molecule of cystine affords a means 

 of readily distinguishing it from other amino acids. The sulphur 

 is held in loose combination and is partially evolved as hydrogen 

 sulphide on boiling with alkali : 



On dissolving some cystine in caustic alkali, or on adding caustic 

 alkali to a solution of cystine, to which a drop of lead acetate solution 

 has been added and boiling, a brownish or black precipitate of lead 

 sulphide is formed. 



Taurine. 



Taurine, or aminoethyl sulphonic acid, is an amino acid containing 

 sulphuric acid instead of carboxyl as the acid group. It has not been 

 found as a constituent of proteins, but is probably derived from cystine 

 or cysteine, which is converted by oxidation in the animal body into 

 taurine : 



CH, . SH CH a . SO 3 H CH, . SO 3 H 



CH.NH,-CH.NH 2 - CH,.NH 2 + CO, 



COOH COOH 



Cysteine. Cysteic Acid. Taurine. 



Taurine has been isolated from the lungs and kidneys of oxen and 

 from the muscles of invertebrates. In combination with cholalic acid 

 as taurocholic acid it is present in bile. 



Preparation. 



Taurine is most easily prepared from ox bile by boiling it for some hours with 

 dilute hydrochloric acid. The filtrate from the insoluble anhydrides of the bile 

 acids is concentrated on the water-bath to a small volume and filtered whilst 

 hot from sodium chloride, etc. The solution is evaporated to dryness and 

 the residue dissolved in 5 per cent, hydrochloric acid. It is precipitated 

 from solution by adding i o volumes of 95 per cent, alcohol. The crystals are 

 purified by solution in acid, precipitation by alcohol and recrystallisation from 

 water. 



