CH. IV.] TRYPTOPHANE. 87 



It crystallises from aqueous alcohol in platelets, which melt 

 at 251 252 C. 



Even in very dilute solution histidine gives a volu- 

 minous white precipitate with phosphotungstic acid. It 

 is also precipitated by mercuric chloride in alkaline solution 

 and by ammoniacal silver nitrate solution. 



86. Totani's reaction for histidine. To a small knife point 

 of the hydrochloride in a small beaker, add 2 cc. of 10 per cent, 

 sodium carbonate. Dissolve a rather larger quantity of diazo- 

 benzene-sulphonic acid in 4 cc. of 10 per cent, sodium carbonate in a 

 test-tube, and add this to the solution in the beaker. A dark red 

 colour is produced (Primary colouration). Make the solution 

 distinctly acid with strong hydrochloric acid. The solution becomes 

 orange coloured. Add zinc dust and allow the reduction to proceed 

 for about 15 minutes. Transfer a few cc. of the clear, colourless, 

 supernatent solution to a test-tube, and render the solution alkaline 

 by the addition of 25 per cent, ammonia. A characteristic 

 golden yellow colouration is produced, which is permanent for 

 a considerable time (Secondary colouration). 



NOTE. Tyrosine gives a primary colouration that is identical with that 

 described above. The secondary colouration is, however, a bright rose red, 

 which gradually changes to a reddish brown. The reaction should be tried 

 with the two substances simultaneously. 



Tryptophane. 



87. Preparation. 



A. Digestion of the Casein. 



(i.) Weigh out 200 grams, of commercial casein.* 



(ii.) Gradually stir this into i litre of cold distilled water in a 

 large beaker, or enamelled vessel, avoiding the forma- 

 tion of lumps as far as possible. 



(iii.) Transfer the viscous mass to a Winchester quart bottle 

 by means of a large funnel with a short wide neck. 



(iv.) Wash out the mixing vessel with a jet of hot water and 

 transfer this, to the bottle, washing the funnel down 

 with some more hot water. 



* " Laitproto No. 6 " (prepared by Casein, Ltd., Culvert Works, Batter- 

 sea, London, S.W.) can be recommended as a suitable preparation. 



