Separation of Leucin from Valin. 



55 



acetate used for precipitation. The valin is obtained analytically 

 pure by freeing the filtrate from lead with H^S, evaporating to dry- 

 ness, and washing with absolute alcohol. A slight amount of valin 

 dissolves, but is regained by evaporating the washings. 



The following is a typical separation, the material being a por- 

 tion obtained by tryptic digestion of casein and fractional distilla- 

 tion of the amino-acid esters. 12.546 g. of the mixture was used. 

 Analysis showed 52.79 per cent. C, 9.55 per cent. H. The 

 mixture was suspended in 80 c.c. of boiling water. The flask was 

 removed from the flame and 20 c.c. of concentrated aqueous 

 ammonia added. The flask was loosely stoppered, and shaken 

 gently until the acids were dissolved. The leucin was then pre- 

 cipitated with 25 c.c. of M/i lead acetate. The cooled solution 

 was filtered, and the precipitate washed with 50 c.c. of dilute 

 ammonia. 8.955 g. of lead salt, equivalent to 5.025 g. of leucin, 

 and 7.322 g. of valin were obtained analytically pure, making 

 12.347 g. from the original 12.546 g. Analytic data: 



Lead salt: (i) 44.25 per cent. Pb ; (2) 44.36 per cent. Pb. 

 Calculated for Pb(CgH(202N)2, 44.29 per cent. Pb. 



Valin: 51.44 percent. C; 9.42 per cent. H. Calculated for 

 CjHjjOjN, 51.24 per cent. C; 9.47 per cent. H. 



The specific rotation of the valin was [a]^° d -f 26.51°. 

 The pure active substance has the rotation -f- 28.8°. The product 

 was partially racemized, the usual result of long tryptic digestion. 



The lead-leucin salt contains a mixture of leucin and isoleucin. 

 Levene and Jacobs ' have shown that these isomers can be readily 

 separated in the absence of valin. The complete separation of the 

 two leucins from valin, therefore, renders the systematic separation 

 of all three comparatively easy of accomplishment. This is of im- 

 portance, not only for protein analysis, but also for the preparation 

 of pure active valin and isoleucin, a task which has hitherto been 

 extremely difficult. 



The work will be reported in full in the Biochemische Zeitschrift. 



■ Levene and Jacobs : Bioch. Zeitschr., 9, 231, 1908. 



