46 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



is stoppered and shaken so as to dissolve the amino acids ; if necessary 

 the solution may be again heated. 



4 c.c. of IT M lead acetate solution (sp. gr. 1*254 at 20) for 

 each gram of leucine and isoleucine are slowly added to the hot solu- 

 tion, which is thoroughly stirred during the addition. The solution 

 is then cooled in ice-water and after one to two hours is filtered 

 through a Buchner funnel, or a Gooch crucible, according to the 

 amount of precipitate. The solid matter is pressed down so as to 

 remove the mother liquor as completely as possible, and it is washed 

 firstly with 90 per cent, alcohol, then with ether, and dried in vacua 

 over sulphuric acid. 



It is curious that the presence of valine facilitates the separation of 

 the lead salts of the leucine isomers. 



If the proportion of isomers : valine be less than 2 : I the pre- 

 cipitation is not so complete. In these cases less lead acetate solution 

 (3 7 c.c.) should be taken and the filtrate concentrated in vacuo till the 

 percentage of valine reaches 10. Ammonia is again added and the 

 precipitate treated as before. It is preferable to treat the filtrate once 

 more in the above manner after the valine has been separated. 



ii. Valine. 



The filtrate is freed from lead by means of hydrogen sulphide, and 

 the solution, filtered from lead sulphide, is evaporated to dryness. The 

 dry residue is treated with an alcohol-ether mixture (3 : i) to extract 

 acetic acid and ammonium acetate. The small amount of valine 

 which dissolves is recovered by again evaporating to dryness and 

 extracting with alcohol and ether. 



Pure valine generally remains ; it is identified by recrystallisation 

 from water, elementary analysis, and rotation in 20 per cent, hydro- 

 chloric acid of [a] D = + 28'8; a rotation of 26-28 is generally found 

 as racemisation occurs in the process. 



If analysis shows leucine to be present the above treatment must 

 be repeated. 



