14 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



Elementary analysis of the free acid, or of its hydrochloride, determines 

 its identity and its weight gives the amount in the protein. 



Aspartic Acid. A portion of the aspartic acid, after separation 

 from phenylalanine ester and after hydrolysis by baryta, may separate 

 as barium salt ; this is the barium salt of racemic aspartic acid. The 

 remainder is isolated, when the glutamic acid has been removed as 

 hydrochloride, by boiling with lead hydroxide and treating with 

 hydrogen sulphide to remove hydrochloric acid and lead respectively, 

 and by crystallising from water. It maybe characterised by conversion 

 into its copper salt, or by analysis, and is estimated by its weight. 



Serine. Its ester is contained in the fractions which distil between 

 100 and 130 at 0*5 mm. The mixed esters contained in this fraction 

 are treated with a small quantity of water and then with five to six 

 volumes of petroleum ether, which precipitates serine ester as an oil ; 

 the oil is then shaken up with petroleum ether to remove admixtures as 

 far as possible and is hydrolysed with baryta water. On removal of 

 the baryta it crystallises when the solution is concentrated, and it is 

 purified by treatment with alcohol, which dissolves other substances 

 which are also present, and recrystallisation from water. Its /3-naph- 

 thalene sulpho-derivative, 



/CH 2 OH /CH 2 OH 



C 10 H 7 SO 2 C1 + H 2 N . CH( = HC1 + C 10 H 7 SO 2 . NH . CH( 



\COOH \COOH 



obtained by shaking it in alkaline solution with ^-naphthalene sulpho- 

 chloride, is very suitable for its characterisation. 



The Isolation of Oxyproline. 



Only in a, few cases has this compound been isolated from the 

 products of hydrolysis of proteins, since its separation is extremely 

 laborious. It can only be effected after all the other amino acids have 

 been removed by crystallisation and by the ester method, and after the 

 diamino acids have been precipitated by phosphotungstic acid. From 

 the last mother-liquors it is obtained by crystallisation, and is best 

 identified in the form of its ^-naphthalene sulpho-derivative. 



The Isolation and Estimation of Tryptophane. 



Tryptophane is not obtained in any large amount by the hydrolysis 

 of proteins by acids and is best prepared by the action of trypsin. Ac- 

 cording to Hopkins and Cole, the protein is digested in alkaline solu- 

 tion by trypsin, until the solution gives a maximal coloration when 

 tested with bromine water ; the solution is then acidified, boiled and 



