14 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



together was described by Morner in 1901. The protein hair, keratin 

 from horn, egg-shells, etc. was boiled with five times its quantity of 

 13 per cent, hydrochloric acid under a reflux condenser on a water- 

 bath for six to seven days. The solution was then decolorised with 

 charcoal and evaporated in vacuo, and the residue dissolved in 60- 

 70 per cent, alcohol. The two acids then crystallised out on neutralis- 

 ing with soda, and were separated by fractional crystallisation from 

 ammonia ; if much tyrosine was present it separated out first, but if 

 cystine exceeded tyrosine in quantity this compound crystallised out 

 first ; the remainder was only separated with difficulty. 



Embden separated the mixture of the two acids by means of 

 very dilute nitric acid, in which tyrosine is very easily soluble, but 

 cystine with difficulty. 



Their separation may also be effected by precipitation with mer- 

 curic sulphate in 5 per cent, sulphuric acid solution in which the 

 mercury compound of tyrosine is soluble (Hopkins and Cole). 



Winterstein, in 1901, showed that cystine was precipitated by 

 phosphotungstic acid, but the observation seems to have been over- 

 looked. Cystine and tyrosine are best separated by means of this 

 reagent The mixture is dissolved in 5 per cent, sulphuric acid and 

 treated with excess of phosphotungstic acid solution. Cystine phos- 

 photungstate generally separates out in a crystalline condition. From 

 this precipitate the cystine can be obtained by the usual method (see 

 under diamino acids), but a large excess of baryta must be avoided as 

 cystine is readily decomposed by alkalies. Cystine crystallises out on 

 neutralising the filtrate from the barium phosphctungstate. Winter- 

 stein decomposed the phosphotungstate with hydrochloric acid. The 

 precipitate is made into a paste with water, placed in a separating 

 funnel and treated with small quantities of concentrated hydrochloric 

 acid. Ether is added and the mixture well shaken. At first an 

 emulsion is formed, but on adding more ether and acid and thoroughly 

 shaking a clear ethereal solution of phosphotungstic acid settles to the 

 bottom ; if this does not occur, decomposition is not complete and 

 more acid must be added and the shaking repeated. The middle layer 

 containing the cystine hydrochloride is separated ; the other layers are 

 treated once more in the same way and the middle layers are combined. 

 Ether is removed from the acid solution by warming on the water- 

 bath and the cystine is separated by exactly neutralising with soda. 



Folin prepares cystine from wool by boiling it with concentrated 

 hydrochloric acid in the proportion of 100 grammes wool to 200 c.c. 



