50 CHEMISTRY OF THE PROTEIDS CHAP. 



They suggest that figures lying between these extremes are obtained 

 owing to an admixture of racemic ty rosin. 



Tyrosin is very slightly soluble in water. It is nearly tasteless 

 like chalk, and in this respect resembles phenyl-amino-acetic acid 

 (C 6 H 5 . CH(NH 2 ) . COOH), (Fischer). 



Tyrosin gives a number of characteristic colour reactions : 



(a) Millon's Reaction. If one boil a fluid containing tyrosin with 

 Millon's reagent, a solution of mercurous nitrate in dilute nitric acid 

 (see p. 7), the fluid turns red, and the depth of colour, from 

 pink to a reddish black, depends on the amount of tyrosin present. 

 This reaction is given by all benzene-derivatives in which one hydrogen 

 atom has been replaced by a hydroxyl group. Nasse l has made a 

 special study of the behaviour of different benzene derivatives towards 

 Millon's reagent. Substitution of the other benzene hydrogen atoms 

 by halogens destroys _. Millon's reaction. 2 Not only tyrosin, but 

 albumin itself gives the reaction (see p. 7). 



(b) -Piria's Reaction. 3 Tyrosin is added in a solid form to a small 

 quantity of sulphuric acid ; this mixture is then neutralised with 

 barium carbonate, and is filtered. On adding to the solution of barium- 

 tyrosin sulphate a very dilute solution of ferric chloride, a deep violet 

 colour results, owing to the formation of iron-tyrosin sulphate. 



(c) Morner's Reaction. 4 Mix formaldehyde 1 part, water 45 parts, 

 and concentrated sulphuric acid 55 parts. When this solution, 

 which is permanent, is boiled with tyrosin, a green colour is obtained. 



(d) Bertrand's Reaction? Very characteristic is also the darkening 

 of tyrosin which is produced by vegetable oxydases, the so-called 

 tyrosinases. Extract mushrooms (Uussula nigricans) with water and 

 precipitate with alcohol. The precipitate dissolved in water colours 

 tyrosin in a few hours first red, then black (see also p. 89). 



(e) Ehrlich-Pauly's diazo-reaction has already been described on 

 p. 9. 



The largest amount of tyrosin is obtainable from the fibroin of silk 6 

 and from zein, 7 next from keratin, casein, and protalbumose (compare 

 tables on pp. 70-75). 



1 0. Nasse, Pfliigers Arch. f. d. ges. Physiol. 83. 361 (1901). 



2 F. Blum and W. Vaubel, Journ.f. prakt. Chem. (2) 56. 393 (1897) ; (2) 57. 365 

 (1898) ; A. Oswald, Hofmeisters Beitr. 3. 391 (1903). 



3 R. Neumeister, Lehrbuch d. physiol. Chem. 2. Aufl. Jena, G. Fischer, 1897, 

 p. 250. 



4 C. T. Morner, Zeitschr.f. physiol. Chem. 37. 86 (1902). 



5 G. Bertrand, Compt. rend. 123. 463 (1896). 



6 E. Fischer and A. Skita, Zeitschr.f. physiol. Chem. 33. 177 (1901). 



7 F. Kutscher, ibid. 38. Ill (1903). 



