CHEMICAL COMPOSITION OF PROTEIN MOLECULE 31 



(6) Aspartic Acid. 



The esters in the aqueous solution from which the phenylalanine 

 ester has been extracted with ether are saponified by boiling with 

 baryta ; the solution of baryta is prepared by dissolving twice the 

 quantity of barium hydrate to that of ester in sufficient hot water, 

 filtering, and allowing to cool. The clear solution is poured off from 

 the crystals and to it is added the solution of esters. Hydrolysis is 

 then effected by heating for two hours on the water-bath. The solu- 

 tion is allowed to stand for several days to allow the barium salt of 

 racemic aspartic acid to crystallise out. 



The barium aspartate is decomposed with sulphuric acid, the- 

 barium sulphate filtered off, and the excess of sulphuric acid quantita- 

 tively removed with baryta. Pure aspartic acid crystallises out from 

 the solution on evaporation. It is identified by analysis and by the 

 analysis of its copper salt. 



The remainder of the aspartic acid is isolated after removal of 

 glutamic acid as hydrochloride (c). 



The solution is evaporated in vacua to remove as much hydrochloric 

 acid as possible. The residue is dissolved in water and boiled with 

 yellow lead oxide until a test portion of the cold solution no longer 

 gives a reaction for chlorine. The filtered solution is freed from lead 

 by hydrogen sulphide and the filtrate from lead sulphide is evaporated 

 to a small volume, when aspartic acid crystallises out. The mother 

 liquor contains principally serine, but more aspartic acid and other 

 products are also present. 



Note. Osborne and Liddle have observed that an intermediate fraction between fractions 

 III. and IV. containing aspartic ester and leucine ester and possibly also phenylalanine ester 

 may distil. The separation of leucine and aspartic acid (or glutamic acid) is impossible by 

 fractional crystallisation. The fraction should be treated as described and the leucine separ- 

 ated from the aspartic acid by neutralising with soda and crystallising. Leucine results. 

 On acidifying the filtrate and again crystallising the aspartic acid is obtained. 



