ISOLATION AND ESTIMATION OF TYROSINE i; 



A. THE MONO-AMINO ACIDS. 

 Tyrosine. 



(i) Isolation and Gravimetric Estimation. 



Hydrolysis by sulphuric acid possesses one great advantage over 

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

 easily removed by baryta. 



The protein is 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, is diluted with twice its volume of water and 

 neutralised with barium carbonate, or a strong solution of baryta, the 

 excess of which is then quantitatively removed by dilute sulphuric 

 acid. The solution, together with the water used in thoroughly wash- 

 ing the precipitate of barium sulphate, is then evaporated down, 

 until the compound crystallises out. It is filtered off, the filtrate is 

 concentrated, and further crops of crystals are removed until the 

 mother liquor no longer gives Millon's reaction for tyrosine. The 

 amount of cystine in most proteins is so small that the product 

 generally consists only of tyrosine. It is purified by recrystallisation 

 from water, decolorisation of the solution being effected by charcoal. 

 The yield of tyrosine so obtained is the measure of its amount in the 

 protein. 



On account of the insolubility of tyrosine and the difficulty of 

 filtering and completely washing the barium sulphate precipitate in 

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

 Teruuchi [1906], in the case of silk, hydrolysed the protein with 

 hydrochloric acid, the greater part of which was then removed by 

 evaporating several times in vacuo after diluting the concentrated 

 solution with water ; the remainder of the hydrochloric acid was then 

 estimated in a small aliquot portion, and the main bulk neutralised 

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

 stallised out and was purified by recrystallisation from water. 



Abderhalden [1912] shortened the process by hydrolysing the silk 

 by boiling for three hours with three times its quantity of concentrated 

 hydrochloric acid, evaporating the solution to dryness repeatedly in 

 vacuo to remove the hydrochloric acid, dissolving the residue in water 

 and passing in ammonia gas, or dissolving in ammonia, again evaporat- 

 ing in vacuo to dryness and treating the residue with cold water. 



PT. I. 2 



