32 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



(c) Glutamic Acid. 



The filtrate from the barium aspartate is exactly freed from barium 

 by sulphuric acid and the solution is evaporated to dryness in vacua. 

 The residue is dissolved in water, the solution decolourised, if neces- 

 sary, by boiling with charcoal and the glutamic acid is precipitated 

 as hydrochloride by passing in dry gaseous hydrogen chloride. A 

 further quantity of glutamic acid hydrochloride may be obtained from 

 the mother liquor by concentration and similar treatment. Practically 

 the whole of the glutamic acid present in the protein is thus obtained 

 as hydrochloride. The larger portion is separated directly, before 

 the mixture of amino acids is esterified. 



Glutamic acid is obtained from the hydrochloride by treatment with 

 the calculated quantity of caustic soda to combine with the hydro- 

 chloric acid and by crystallisation from water, in which it is soluble, 

 when pure, with some difficulty. Elementary analysis of the free 

 acid, or of its hydrochloride, determines its identity and its weight 

 gives the amount in the protein. 



(d) Serine. 



It is most difficult to isolate serine and obtain it in a pure state. 

 The solution from which the active aspartic acid has crystallised 

 out is neutralised, if acid, with caustic soda and concentrated. Serine 

 crystallises out in crusts of monoclinic crystals, and is identified by its 

 melting-point of 240 and elementary analysis. 



Its /3-naphthalene sulphonyl-derivative, 



/CH 2 OH /CH 2 OH 



C 10 H 7 SO ls Cr+ H 2 N . CH( = HC1 + C 10 H 7 SO 2 . NH . CH< 



\COOH \COOH, 



which is prepared by shaking in alkaline solution with /9-naphthalene 

 sulphonyl-chloride, serves for the isolation of the remainder from the 

 filtrate. This compound is very suitable for its characterisation. 

 (M.P. = 214 C. corr.) 



