CHEMICAL COMPOSITION OF PROTEIN MOLECULE 13 



carried out by F. Ehrlich, who makes use of the different solubility of 

 the copper salts of these two amino acids in methyl alcohol. Levene 

 has also employed this method. 



Proline. This is the only product of hydrolysis obtained from an 

 ester fraction which is soluble in alcohol ; it is also much more easily 

 soluble in water than the other products with which it is present and 

 therefore is somewhat easily separated, as it remains in the mother- 

 liquor after these have crystallised out. The solution, in which it is 

 contained, is evaporated to dry ness and extracted with absolute alcohol ; 

 the combined alcoholic extracts from the several fractions are evaporated 

 to dryness and taken up by absolute alcohol several times, so as to 

 remove small amounts of the other amino acids, which, though insoluble 

 in alcohol, are dissolved when proline is present. 



As thus obtained, the proline is a mixture of the optically active 

 and the racemic forms ; these are separated by conversion into their 

 copper salts and treatment with absolute alcohol which dissolves that 

 of the optically active proline. Their purification is easy, and a deter- 

 mination of the water of crystallisation and of the copper establishes 

 the identity of the compound. The phenylhydantoine derivative may 

 also be used for this purpose. 



Phenylalanine. Phenylalanine is separated in the form of its ester 

 from those of serine and glutamic and aspartic acids. The mixed esters 

 are dissolved in water, and if a large amount of phenylalanine ester be 

 present, it separates in the form of oily drops, but in any case the 

 aqueous solution is extracted with ether. The ester, obtained after 

 removal of the ether, is hydrolysed by evaporation with concentrated 

 hydrochloric acid, and the resulting phenylalanine hydrochloride is puri- 

 fied by crystallisation from strong hydrochloric acid. By evaporating its 

 aqueous solution with ammonia, treating with ice-cold water to dissolve 

 the ammonium chloride, and precipitating it from its hot aqueous 

 solution by alcohol, a pure preparation of phenylalanine is generally 

 obtained, from the weight of which its percentage amount in the protein 

 is calculated. 



Glutamic Acid. The greater part of the glutamic acid is isolated 

 as hydrochloride before the mixture of amino acids is esterified. It 

 is contained with aspartic acid ester in the aqueous solution after the 

 phenylalanine ester has been extracted by ether, and it is separated 

 from aspartic acid, after hydrolysis by baryta, by conversion into its 

 hydrochloride ; from this it is obtained by treatment with the calculated 

 quantity of soda to combine with the hydrochloric acid and by crystal- 

 lisation from water, in which it is soluble with some difficulty. 



