CHEMICAL COMPOSITION OF PROTEIN MOLECULE 39 

 VI. Estimation and Isolation of Lysine. 



The lysine is contained in the filtrate from the precipitate of the 

 silver salts of arginine and histidine. 



The solution is acidified with sulphuric acid and freed from silver 

 by hydrogen sulphide ; the filtrate and washings from the precipitate 

 of silver sulphide and barium sulphate, which is treated in the usual 

 manner, are evaporated down to 500 c.c. Sulphuric acid ; . then added 

 until the content is 5 per cent., and the lysine is precipitated by not 

 too large an excess of phosphotungstic acid. This is added until a 

 portion of the clear liquid on the further addition of the reagent 

 remains clear for ten seconds. After twenty-four hours the precipitate 

 of lysine phosphotungstate is filtered off by suction and washed with 

 5 per cent, sulphuric acid by stirring up in a mortar. After making 

 up the filtrate and washings to a definite volume an estimation of the 

 substances not precipitated may be made in an aliquot part by the 

 Kjeldahl method. 



The lysine phosphotungstate is made into a uniform suspension 

 with water and poured into boiling water. A hot saturated solution of 

 baryta is added until the solution is strongly alkaline and contains ex- 

 cess of baryta. The precipitate of barium phosphotungstate, which is 

 formed, is filtered off and boiled out several times with baryta and then 

 with water. The alkaline solution is freed from bartya by means of 

 carbon dioxide, concentrated, filtered, and evaporated on the water-bath 

 nearly to dryness. .Vater is then added, the barium carbonate filtered 

 off and washed, and the solution once more evaporated, aK ir which it is 

 made up to a definite volume and the lysine estimated in an aliquot 

 portion by Kjeldahl's method. 



The lysine is separated from the remainder of the solution as picrate. 

 The solution is evaporated down in a porcelain basin to dryness, and a 

 small quantity of alcohol is added to the sticky residue. It is then treated 

 with a saturated solution of picric acid in alcohol until no further pre- 

 cipitation of picrate occurs. After twenty-four hours this precipitate is 

 filtered off and washed with a small quantity of absolute alcohol ; 

 it is then recrystallised by solution in boiling water, filtering if 

 necessary, and evaporating to a small volume, when lysine picrate, 

 C 6 H 4 N 2 O 2 . C 6 H 2 (NO 2 ) 3 OH, crystallises in needles on cooling ; these are 

 filtered off, washed with alcohol, dried and weighed. 



The last portions of the lysine in the mother liquor from the picrate 

 can be obtained by acidifying with sulphuric acid, extracting the picric 

 acid with ether, precipitating as phosphotungstate, and repeating the 

 above process for obtaining lysine picrate. 



