CHEMICAL COMPOSITION OF PROTEIN MOLECULE i> 



The Isolation and Estimation of the other Monoamino Acids. 



For the preparation and estimation of the monoamino acids, hydro- 

 lysis by hydrochloric acid is more convenient than that by sulphuric 

 acid. It was formerly carried out in the presence of stannous chloride 

 (Hlasiwetz and Habermann) in order that the solution should remain 

 colourless, instead of becoming dark brown, but this addition is not 

 essential, as was shown by Cohn, and was not used by E. Fischer in 

 his researches. Hydrolysis by hydrochloric acid is carried out by 

 heating the protein with three times its quantity of concentrated 

 hydrochloric acid of specific gravity 1*19, when it gradually passes 

 into solution, if the flask in which it is contained be occasionally 

 shaken and slightly warmed, and then boiling under a reflux conden- 

 ser for ten to twenty-four hours or longer, depending on the particular 

 protein, and until the biuret reaction has completely disappeared. 

 The solution, which may become at first violet, becomes finally of a dark- 

 brown colour, and a portion of the hydrochloric acid is evolved as gas j 

 it is filtered from humin substances (secondary products probably arising 

 from tryptophane and carbohydrate) and fatty material through a 

 Buchner funnel covered with linen, and well washed with water. 



(i) Isolation of Glutamic Acid as Hydrochloride. 



The solution of amino acids in 25 per cent, hydrochloric acid is con- 

 centrated in vacua to a small volume, and glutamic acid, if present in 

 any large amount, is removed as its hydrochloride by saturating the 

 solution with dry gaseous hydrochloric acid and allowing to stand at 

 o C. for some days, when glutamic acid hydrochloride crystallises out. 

 This occurs in the case of caseinogen and certain vegetable proteins, 

 which contain from 10-40 per cent, of this amino acid. The glutam- 

 ic acid hydrochloride is filtered off after adding an equal volume of 

 ice-cold alcohol, redissolved in water and boiled with baryta to re- 

 move ammonia. The barium is removed with sulphuric acid and the 

 glutamic acid is again precipitated as hydrochloride by saturating the 

 solution with gaseous hydrogen chloride. The mother liquor on further 

 concentration may give further crops of glutamic acid hydrochloride. 

 These are treated in the same way as the first crop. 



PT. I. 



