CHEMICAL BASIS OF THE ANIMAL BODY. 149 



Preparation, (i) From horn-shavings by prolonged boiling with 

 sulphuric acid, 5 of acid to 13 of water. The resulting fluid is 



FIG. 12. LEUCIN CRYSTALS. (Krukenberg.) 



neutralised by baryta, and filtered, the excess of baryta removed 

 by the cautious addition of dilute sulphuric acid, and the final filtrate 

 concentrated to crystallisation. It is separated from tyrosin by 

 repeated crystallisation taking advantage of the great solubility of 

 leucin and the slight solubility of tyrosin. (ii) From the products 

 of the tryptic (pancreatic) digestion of proteids. After prolonged 

 digestion, using thymol and salicylic acid to prevent putrefaction, 

 the fluid is filtered, moderately concentrated and set aside to crystallise ; 

 by this means a large part of the accompanying tyrosin is removed. 

 The filtrate is now further concentrated, treated with excess of hot 

 alcohol, which precipitates the peptones, and filtered while hot. If much 

 leucin is present a large part of it crystallises out on cooling the alcoholic 

 filtrate, and the rest on concentrating by slow evaporation. There is a 

 large loss of leucin by both the above methods and the resulting product 

 is far from pure. To obtain pure leucin it should be synthetised by the 

 action of ammonia on a-bromcaproic acid 1 . 



Even an approximately quantitative separation of leucin from solutions where it 

 is mixed with other substances, e.g. an extract of tissues or a digestive mixture, is 

 a matter of great difficulty. Advantage may in some cases be taken of its behaviour 

 towards hydrated oxide of copper, with which it forms a compound 2 . 



For ordinary practical purposes the microscopic appearance of the 

 crystals affords the most convenient means for recognising leucin, and 

 in this way very minute traces may be determined with certainty. 

 The confirmation of the clue thus afforded by the application of chemical 



1 Kilmer, Jn. f. prakt. Ghent. (2) Bd. i. (1870), S. 6. 



2 Hlasiwetz u. Habermann, Ann. d. Chem. u. Pharm. Bd. CLXIX. (1873), S. 150. 



