24 CHEMISTRY OF THE PROTEIDS CHAP. 



nature, combine to form salts and thereby keep one another mutually in 

 solution. 1 The solubilities of isolated, individual ammo-acids, of many 

 of their salts, and of their derivatives are frequently but low, but as 

 soon as other homologous compounds are present, their solubilities are 

 increased enormously. 2 



Even crystallisation 'is no guarantee as to the purity of an amino- 

 acid, because leucin and amino-valerianic acid and their cuprates 

 have a great tendency to crystallise out together either as double 

 salts or as mixed crystals. 2 Ritthausen 3 used in his researches the 

 method of displacing the bases of the amino-acids with barium 

 hydrate, and then precipitating the slightly soluble barium salts of, 

 e.g., asparagin and glutamin with alcohol. Kutscher 4 adopted the 

 plan of precipitating the bases of the amino-acids with phospho- 

 tungstic acid, but he could not get the acid residue to crystallise out. 

 In the case of glutaminic acid Kutscher obtained a crystalline product 

 by removing the bases ; allowing the tyrosin and part of the leucin 

 to crystallise out ; precipitating the glutaminic acid as a zinc salt ; 

 removing the zinc and finally evaporating the remaining fluid till 

 crystals appeared. Aspartic acid has been estimated by Hlasiwetz 

 and Habermann as a silver salt, and by Ritthausen as a copper salt, 

 but these methods are very uncertain and accompanied by great loss, 

 and therefore only applicable if a substance is very rich in asparagin. 

 Kutscher's 5 method of forming silversalts of the amino-acids has not 

 yet been put sufficiently to the test. He adds to a solution of amino- 

 acids a 20 per cent solution of silver nitrate, and subsequently barium 

 hydrate, when the slightly soluble silversalts of leucin and of glycocoll 

 separate out, while the salts of alanin and of amino-valerianic acid 

 remain in solution. 



The first practical method for the isolation of amino-acids we owe 

 to Emil Fischer, 6 who converts the amino-acids into their respective 

 ethyl-esters, according to the method of Curtius, 7 and then separates 

 the individual esters, by means of fractional distillation, under the 

 lowest attainable pressure. The only ester which is not distilled but 

 is allowed to crystallise out is glycocoll -ester. 8 From the residue 



1 F. Kutscher, Zeitschr. f. physiol. Ghem. 28. 123 (1899) ; F. Hofmeister, Liebig's 

 Ann. 189. 6 (1877). * a E. Fischer, Zeitschr. f. physiol Ghem. 33. 151 (1901). 



3 H. Ritthausen, Journ. f. prakt. Ghem. 107. 218 (1869). 



4 F. Kutscher, Zeitschr. f. physiol. Ghem. 38. HI (1903). 



5 F. Kutscher, Sitz.-Ber. d. Berliner Akad. d. Wiss., phys.-math. Kl., 29 May 1902 ; 

 Zeitschr. f. physiol. Ghem. 38. Ill (1903). 



6 E. Fischer, Ber. d. deutsch. chem. Ges. 34. I. 433 (1901) ; Zeitschr. f. physiol. 

 4Chem. 33. 151 (1901) ; E. Fischer and E. Abderhalden/?M2. 36. 268 (1902). 



7 Th. Curtius and F. Gobel, Journ. f, prakt. Chem. (2), 37. 150 (1888). 

 V 8 E. Fischer, Zeitschr. f. physiol. Chem. 35. 229 (1902). 



