26 THE CHEMICAL CONSTITUTION OF THE PROTEINS. 



Isolation of Glycine, Alanine, Valine, Leucine, and Isoleucine. 



These five amino acids are present together in varying proportions 

 in the residues which are insoluble in absolute alcohol. Their 

 separation is only effected with great difficulty, and the procedure de- 

 pends very largely upon which amino acids are present in the several 

 fractions. 



In each case the residue is dissolved in water and, if necessary, the 

 solution is decolourised by boiling with charcoal. The aqueous solu- 

 tions are concentrated and fractionally crystallised ; the final mother 

 liquor is evaporated to dryness. Each fraction is dried and weighed. 

 Indications of the constituents of each fraction may be obtained : 



1. By elementary analysis of the carbon, hydrogen, and nitrogen 

 content. 



2. By determining the melting-point ; the substance must be 

 rapidly heated. Glycine melts at 240, alanine about 297, leucine 

 about 300, and valine about 315. 



3. By the taste. Glycine and alanine have a sweet taste ; valine 

 is less sweet and it leaves a bitter after-taste ; leucine is insipid and 

 slightly bitter. 



The residues from fraction I. will contain chiefly glycine and al- 

 anine; from fraction II. valine, leucine and isoleucine; from fraction 

 III. leucine and isoleucine. 



Separation of Valine from Leucine and Isoleucine. 



These three compounds are the most difficult to separate from each 

 other. Their separation has really only been accomplished by chance. 



In those cases where the isolation of the individual substance has 

 succeeded it has been effected by the fractional crystallisation of the 

 amino acids themselves and of their copper salts and by the different 

 solubility of the compounds in methyl alcohol. Valine is soluble in 

 methyl alcohol, isoleucine is insoluble in the cold solvent, but soluble 

 when hot. On cooling the solution, however, the presence of valine 

 prevents its separation. The copper salt of leucine is very insoluble, 

 but the mixed copper salts are relatively soluble. Leucine and iso- 

 leucine were first separated by F. Ehrlich by the different solubility 

 of their copper salts in methyl alcohol ; that of leucine is insoluble. 

 The separation is most tedious and not at all satisfactory. 



In order to give us more information about the constituents of this 



