CHEMICAL COMPOSITION OF PROTEIN MOLECULE 7 



Hydrolysis by sulphuric acid possesses one great advantage over 

 that by hydrochloric acid, as it can be subsequently completely removed 

 by baryta. This method was employed by Fischer for obtaining 

 tyrosine and diaminotrioxydodecanic acid from proteins, such as 

 caseinogen, which contains very little cystine. The protein was 

 hydrolysed by boiling with five to six times its quantity of 25 per cent, 

 sulphuric acid for twelve to fifteen hours ; the solution, after filtration, 

 was diluted with twice its volume of water and neutralised with barium 

 carbonate, or a strong solution of baryta, the excess of which was then 

 removed by dilute sulphuric acid. The solution, together with the 

 water used in thoroughly washing out the precipitate of barium sulphate, 

 was then evaporated down, until these acids crystallised out. They 

 were separated from one another by phosphotungstic acid, which 

 precipitated the diaminotrioxydodecanic acid, and they were estimated 

 by weighing, after removal of the phosphotungstic acid by baryta. 



On account of the insolubility of these compounds and the difficulty 

 of filtering and completely washing out the barium sulphate precipitate, 

 in order to abstract from it the whole of the tyrosine, Abderhalden and 

 Teruuchi, in the case of silk, have hydrolysed the protein with hydro- 

 chloric acid, the greater part of which was then removed by evaporation 

 in vacua ; the remainder of the hydrochloric acid was then estimated in a 

 small aliquot portion, and then separated quantitatively by neutralising 

 with the calculated amount of caustic soda. The tyrosine then crystal- 

 lised out, and was purified by recrystallisation from water. 



A new method of determining the presence of tyrosine by bromina- 

 tion was introduced by Horace Brown and employed by Adrian Brown 

 and Millar in 1906 for estimating the rate at which tyrosine is split off 

 from proteins by the action of trypsin. This method might be used for 

 the estimation of tyrosine in proteins ; its non-employment may be due 

 to the fact that tryptophane and also histidine react with bromine and 

 might thus vitiate the result for tyrosine. 



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 essen- 

 tial, 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 



