24 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



Cystine. 



(i) Isolation and Gravimetric Estimation. 



With few exceptions cystine has not been isolated from the pro- 

 ducts of hydrolysis of proteins other than the keratins, and in these 

 cases usually mixed with tyrosine from which it required separation. 

 Morner [1901-2], who first isolated cystine from the products of 

 hydrolysis, heated the protein- hair, keratin from horn, egg-shells, 

 etc. 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. On neutralisa- 

 tion with soda a mixture of cystine and tyrosine separated out. 



Embden [1900] hydro lysed horn by boiling under a reflux with 

 three times its quantity of concentrated hydrochloric acid. The liquid 

 was neutralised to amphoteric reaction and allowed to stand for not 

 more than twenty-four hours. The black pigment (melanin) was 

 filtered off and the filtrate after acidifying was boiled with charcoal. 

 The pale yellow filtrate gave on standing in a cool place a large 

 precipitate of tyrosine and cystine. Further quantities were obtained 

 in the same way on concentrating the mother liquor. 



Some alterations in this procedure were made by Friedmann [1902]. 

 The hydrolysed protein was nearly neutralised with a concentrated 

 solution of caustic soda, decolorised by boiling with large amounts 

 of charcoal, and allowed to cool. Cystine and tyrosine crystallised out. 



A simple method of preparing cystine from wool was described by 

 Folin [1910]. The wool is boiled with concentrated hydrochloric acid 

 in the proportion of 100 grams of wool to 200 c.c. of acid for three to 

 five hours. The hot solution is then neutralised to congo red by adding 

 sodium acetate in the form of crystals. Almost the whole of the cystine 

 separates out on standing. After several hours the precipitate is dis- 

 solved in 3-5 per cent, hydrochloric acid, the solution is boiled with 

 charcoal, and the hot colourless solution is neutralised as before by 

 adding a hot concentrated solution of sodium acetate. Cystine 

 separates out in the characteristic hexagonal plates as the solution 

 cools. No data of the yield were given. 



The mother liquor on dilution and on standing deposits tyrosine. 

 This is most readily purified by dissolving in hydrochloric acid, de- 

 colorising the solution with charcoal, and then neutralising exactly 

 with ammonia, when almost pure tyrosine separates out. 



