CH. IV.] TRYPTOPHANE. 93 



I. Recrystallisation. 



(i.) Transfer the crystals to a small flask, fitted with a reflux 

 condenser. 



(ii.) Add a small amount of 70 per cent, alcohol, and heat in a 

 boiling water bath. Very gradually add 70 per cent, 

 alcohol (down the stem of the condenser) until the 

 crystals have just dissolved. Add a large "knife-point " 

 of decolourising charcoal and heat for five minutes. 



(iiL) Disconnect and rapidly filter through a small hot funnel 

 into a small beaker. Allow to stand for at least an hour. 



(iv.) Filter on the pump, wash with 75, 85, and 95 per cent, 

 alcohol. Dry in a vacuum desiccator, or in a warm oven 

 at 80 to 90 C. There is a considerable loss on recry- 

 stallisation. 



Properties of Tryptophane. It crystallises from aqueous 

 alcohol in white glistening, six-sided plates. It is 

 moderately soluble in cold water, but freely soluble in hot 

 water. It is only sparingly soluble in absolute alcohol. 

 On heating it changes colour at 220, browns at 240, and 

 melts at 252 C. On heating still further, there is first an 

 evolution of carbon dioxide, and then the formation of 

 indol and skatol. 



Tryptophane is optically active, being laevorotatory 

 in aqueous, but dextrorotatory in acid or alkaline solution. 



It gives colour reactions with a great variety of sub- 

 stances. The most important of these is the reaction with 

 glyoxylic and sulphuric acids (see Ex. 23). The investiga- 

 tion of the cause of the similar colour reaction given by 

 proteins led to the isolation of the amino-acid. It also 

 gives colour reactions with most aldehydes in the presence 

 of strong hydrochloric or sulphuric acids containing a trace 

 of an oxidising substance, like ferric chloride. All these 

 reactions are given by tryptophane when it is combined in a 

 protein molecule. 



Free tryptophane gives a red-rose colour when treated 

 with bromine water, the colouring matter being soluble in 



