94 PROPERTIES OF CERTAIN AMINO-ACIDS. [CH. IV. 



amyl or butyl alcohol. This reaction is not given by 

 combined tryptophane. 



Tryptophane is somewhat unstable. It is rapidly 

 destroyed by boiling acids, especially in the presence 

 of carbohydrates, yielding a dark, so-called " humin 

 substance," or " melanin." It is oxidised by certain 

 metallic salts, like copper sulphate, silver nitrate, gold 

 chloride and ferric chloride, yielding indol aldehyde 



iC.CHO 

 HC 



N 



CH NH 



and other substances. On heating on an open water bath 

 in aqueous solution it suffers decomposition. For this 

 reason it is necessary to concentrate in vacuo, or to add 

 considerable quantities of alcohol during the evaporation. 

 It is much more stable in alkaline solutions and, in the 

 presence of other amino-acids, can be boiled with strong 

 baryta for days without appreciable loss. It must be 

 noted, however, that the substance obtained after baryta 

 hydrolysis is racemic (see p. 152). 



In dilute sulphuric acid tryptophane gives a lemon- 

 yellow precipitate with mercuric sulphate, the precipitate 

 being a compound of tryptophane sulphate with mercuric 

 sulphate. The precipitating effect of phosphotungstic acid 

 on tryptophane seems to vary with the quality of the acid ; 

 some samples only give a precipitate in concentrated 

 solutions, others none at all, whilst some preparations pre- 

 cipitate it completely even from relatively dilute solutions, 

 especially in the presence of other amino-acids. It is not 

 precipitated by lead acetate nor by mercuric chloride in 

 neutral solution. 



88. A. To a small knife point of tryptophane dissolved in 

 about 3 cc. of water, cautiously add bromine water. A rose-red or 



