54 



Scientific Proceedings (31). 



action with the butyric acid test indicates either the absence of 

 syphilitic infection or a successful cure of the disease. There is 

 no necessary relation between the Wassermann test and the quan- 

 tity of globulins in the luetic serum. 



23 (361) 



The quantitative separation of leucin from valin. 



By D. D. VAN SLYKE and P. A. LEVENE. 



\_From the Rockefeller InstiUite for Medical Research.'] 

 Of the known amino-acids determined in semi-quantitative esti- 

 mations of final proteolytic products, leucin and its relatives, iso- 

 leucin and valin, have proven unusually difficult to prepare pure in 

 even approximately quantitative amounts. The separation of these 

 substances, because of their close physical and chemical similarity, 

 has offered almost insurmountable difficulties to previous investi- 

 gators. The acids form isomorphous mixtures which are absolutely 

 inseparable by crystallization ; and their esters have so nearly the 

 same boiling points that they cannot be fractionated by distillation. 

 Because of these difficulties, most investigators have not attempted 

 to separate the mixture, but have reported the entire mass as leucin. 

 Fischer ' states that all the figures reported from his laboratory for 

 leucin in protein hydrolyses refer to this mixture. Ehrlich^ has 

 recently reported a method for separating the three substances, but 

 it involves a long process, large losses, and the racemization of the 

 isoleucin and valin. 



We have been able to separate the leucin isomers readily from 

 valin in quantitative amounts. The method, which is very simple, 

 rests on the fact that if a molecular lead acetate solution is added 

 to an ammoniacal solution of the leucin-valin mixture, the leucins 

 are precipitated as analytically pure Pb(CgHj202N)2. If too great 

 an excess of lead acetate is added, a portion of the valin may also 

 be precipitated. Consequently, the mixture is first analyzed, an 

 estimate of the proportion of leucin calculated from the carbon 

 content, and 20 per cent, excess of the theoretical amount of lead 



^ Fischer : Unters. iiber Aminos., Polypeptide, und Protelne, p. 67. 

 2 Ehrlich : Bioch. Zeitschr., 8,'399, 1908. 



