12 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



A. THE MONOAMINO ACIDS. 

 The Isolation and Estimation of Tyrosine and Cystine. 



Two of the amino acids, namely, tyrosine and cystine, are char- 

 acterised by their extremely slight solubility in neutral aqueous solu- 

 tions. They are therefore easily obtained after hydrolysis by acids by 

 neutralising and concentrating the solution, when they crystallise out. 



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 these acids crystallise out. They are 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 these compounds 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, in the case of silk, have hydrolysed the protein with 

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

 evaporating several times in vacua 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. 



When large quantities of protein are under investigation, the 

 removal of the hydrochloric acid, after evaporation in vacuo, is effected 

 by treating the solution with cuprous oxide until it is green in 

 colour, filtering off and washing the cuprous chloride, and removing 



