CHEMICAL COMPOSITION OF PROTEIN MOLECULE 15 



filtered. The clear solution (better after concentrating and filtering off 

 tyrosine, which crystallises out) is acidified with sulphuric acid until it 

 contains 5 per cent., and then mercuric sulphate dissolved in 5 per cent, 

 sulphuric acid is added as long as a precipitate, which contains trypto- 

 phane, cystine and tyrosine, is formed. The precipitate is freed from 

 tyrosine by washing with 5 per cent, sulphuric acid in which the 

 tyrosine compound is soluble, that is, until the washings no longer react 

 with Millon's reagent It is then decomposed by sulphuretted hydrogen, 

 and the solution containing cystine and tryptophane is again acidified 

 with sulphuric acid to 5 per cent, and fractionally precipitated with the 

 mercuric sulphate reagent. The cystine is thrown down first, and 

 filtered off, and then the tryptophane is precipitated. The precipitate is 

 again decomposed by hydrogen sulphide, and the solution, freed from 

 sulphuric acid, is evaporated down, alcohol being continually added to 

 hasten the evaporation and prevent decomposition of the tryptophane, 

 which is estimated by weighing. 



Neuberg and Popowsky, as also Abderhalden and Kempe, have 

 introduced a few alterations in the procedure, such as evaporation in 

 vacuo, and Levene and Rouiller suggested in 1906 that the tryptophane, 

 on account of its proneness to decompose on evaporation of its solution 

 with consequent loss, be estimated colorimetrically ; the mercury sul- 

 phate precipitate is decomposed, and the solution, freed from hydrogen 

 sulphide, is titrated with bromine water in presence of amyl alcohol. 

 Both cystine and tyrosine react with bromine water; the latter can, 

 however, be removed, but for the former a correction has to be made. 

 Up to the present no values concerning the amount of tryptophane in 

 various proteins have appeared, and it will be of interest to see if the 

 values so obtained are very much higher than those obtained by crystal- 

 lisation of the tryptophane. 



II. THE DIAMINO ACIDS. 



The separation and estimation of the three compounds arginine, 

 histidine, lysine is carried out by the method described by Kossel and 

 Kutscher in 1900, which was slightly modified in 1903 by Kossel and 

 Patten. It is based upon the earlier work of Drechsel, Hedin and 

 Kossel, and depends upon the precipitation of arginine and histidine as 

 their silver salts, and of lysine by phosphotungstic acid, and then by 

 picric acid. 



As described by Kossel, Kutscher and Patten the method is as 

 follows : 



I. About 25 to 50 grammes of protein are hydrolysed by boiling with 



