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



89. Preparation of Tyrosine from Casein. 



(i.) Use the mixed mass of calcium phosphate, undigested 



casein, tyrosine, etc., obtained in Ex. 87, B (i.). 

 (ii.) Boil the precipitate with about 250 cc. of water to which 

 has been added 5 cc. of pure sulphuric acid. The tyro- 

 sine dissolves in the acid, whilst a considerable amount of 

 the protein residue remains insoluble. 



(iii.) Filter through a pleated paper, passing the nitrate back 

 through the paper until it is clear. Filtration is apt to 

 be rather slow. 



(iv.) Heat the nitrate on a boiling water bath, and add 10 cc. 

 of strong ammonia. The reaction should now be acid to 

 litmus paper. Cautiously neutralise by the addition of 

 ammonia and allow to cool. Tyrosine crystallises out, 

 generally contaminated with calcium phosphate, etc. 



(v.) Filter off the tyrosine on the pump. Suspend it in about 

 300 cc. of water in a flask, boil, and add 5 cc. of strong 

 ammonia and boil for 15 minutes. 



(vi.) Filter from the calcium phosphate. 



(vii.) Neutralise the fluid with 5 per cent, sulphuric acid and 



allow to stand. 



(viii.) Filter off the tyrosine on the pump, and wash well with 

 . cold water. Wash with a little alcohol, and dry in the 

 steam oven or in a warm incubator. 



90. The separation of tyrosine by fractional precipitation. 



(i.) Treat the filtrate obtained in Ex. 87, C (ii.) with 5 volumes 

 of tap water in a large vessel, mix well, and allow to stand 

 over-night. Owing to the reduction in the concentration 

 of sulphuric acid the tyrosine is precipitated as a com- 

 pound with mercuric sulphate. 



(ii.) Syphon off the supernatant fluid and filter the precipitate 

 on a Buchner. Wash well with water. 



(iii.) Suspend the precipitate in about 200 cc. hot water and 

 decompose by a stream of sulphuretted hydrogen gas. 



(iv.) Boil and filter. Tyrosine is left in solution in dilute 

 sulphuric acid. 



