THE CHEMISTRY OF THE PROTEINS 103 



processes of hydrolysis and esterification, its rotation is gener- 

 ally found too low, and the melting-point of its benzoyl 

 derivative is not quite sharp. 



The presence of valine, or a-aminoisovalerianic acid, is not 

 mentioned in this paper. Its separation from leucine, with 

 which it is generally present, is carried out with difficulty, and 

 depends on the fractional crystallisation of the free acids and 

 their copper salts. 



Leucine, again, as it results on hydrolysis, is partially 

 racemised. Its characterisation is therefore more difficult, but 

 this is carried out by completing the racemisation by heating it 

 with baryta at 170-175 C. for twenty-four hours, and then con- 

 verting it into its hydantoin derivative. Isoleucine is probably 

 mixed with leucine, but no special separation of this isomer was 

 performed by Fischer. 



Proline is the only amino acid which is easily soluble in 

 absolute alcohol. By evaporating the mixture in which it is 

 contained to dryness and extracting with absolute alcohol, 

 it can be easily isolated. This compound, when obtained by 

 hydrolysis, also exists in the racemic and optically active 

 forms ; by conversion into the copper salt the two varieties 

 can be separated by alcohol in which the copper salt of the 

 active form is soluble. The racemic acid is characterised by 

 a determination of the amount of water and the content in 

 copper of its copper salt, the active acid by its hydantoin 

 derivative. 



Phenylalanine is so easily isolated as ester that an analysis 

 of its hydrochloride, obtained by evaporating the ester to dry- 

 ness with hydrochloric acid, is sufficient for its identification. 

 Small quantities can be identified by oxidation to phenylacet- 

 aldehyde, which has a very distinctive smell. 



Serine, on account of the insolubility of its ester in petroleum 

 ether, generally requires no great amount of work to charac- 

 terise it. After hydrolysis by baryta, the solution is concen- 

 trated when it crystallises out, and elementary analysis 

 determines its nature. The /3-naphthalene sulpho-compound 

 may be used, if further characterisation be necessary. 



Aspartic and glutamic acids are separated partially after the 

 hydrolysis of their esters by baryta, when the barium salt of 

 dl-aspartic acid crystallises out on cooling. A further quantity 

 may be isolated by crystallisation of the free acid, but the 



