i8 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



of silver sulphide are evaporated down and made up to I litre; the 

 amount of arginine is calculated from the amount of nitrogen determined 

 in 10 or 20 c.c. of this liquid by Kjeldahl's method. The remainder is 

 freed from sulphuric acid by baryta, the excess of which is removed by 

 carbon dioxide, and the arginine is determined as nitrate C 6 H 14 N 4 O 2 . 

 HNO 3 + | H 2 O by neutralising with nitric acid and evaporating and 

 drying, when it is obtained as a dry white crystalline mass. 



VI. The lysine is obtained from the filtrate from the precipitate of 

 arginine and histidine. This is acidified with sulphuric acid, freed from 

 silver by hydrogen sulphide and evaporated to 500 c.c. Sulphuric acid 

 is then added until the content is 5 per cent, and the lysine is pre- 

 cipitated by phosphotungstic acid. The precipitate is filtered off and 

 thoroughly washed, and is decomposed by baryta ; the barium phos- 

 photungstate formed is filtered off, and the filtrate, freed from baryta 

 by carbon dioxide, is evaporated almost to dryness ; the residue is 

 dissolved in water, filtered from barium carbonate and again evaporated ; 

 it is then treated with small quantities of alcoholic picric acid, so long 

 as a precipitate is formed ; excess must be avoided as lysine picrate 

 is soluble in excess. After some hours it is filtered off and washed 

 with very little absolute alcohol ; it is then dissolved in boiling water 

 and evaporated, when lysine picrate C 6 H 14 N 2 O 2 . C H 2 (NO 3 ) 2 . OH crys- 

 tallises out, and is collected on a weighed filter. The mother-liquor 

 yields a little more lysine picrate, which is treated in the same 

 way. 



The separation and estimation of the two main groups of amino 

 acids can be carried out in one experiment, instead of separately as 

 described. The protein is hydrolysed by sulphuric acid, the tyrosine, 

 cystine and diaminotrioxydodecanic acid are removed by crystallisation, 

 and the diamino acids are precipitated by phosphotungstic acid. From 

 this precipitate they are obtained by decomposition with baryta, and 

 they are then separated by means of their silver compounds by Kossel, 

 Kustcher and Patten's method. The filtrate from the phosphotungstic 

 acid precipitate is freed from the excess of phosphotungstic acid by 

 means of baryta, and the solution is treated by Fischer's ester method 

 for the monoamino acids. 



The combination of the two processes is generally only carried out 

 when the amount of protein available is limited ; they require very 

 different quantities of material ; thus, the diamino acids can be determined 

 in 25 to 50 grammes of protein with considerable accuracy, whereas the 

 monoamino acids can only be determined with fair accuracy when 

 250 to 500 grammes of protein can be used. On the whole, it is not 



