THE AMINO ACIDS 



143 



The benzole acid crystallises out on cooling the solution and is- 

 separated by filtration. 



The presence of glycine in the solution may be shown by adding 

 a slight excess of ammonia, boiling the solution till neutral, and adding 

 a few drops of copper sulphate solution ; the deep blue colour char- 

 acteristic of the copper salt of glycine is formed. 



(4) On adding ferric chloride to a neutral solution of hippuric acid, 

 a reddish-brown precipitate is formed ; this is soluble in hydrochloric 

 acid and the solution will deposit crystals of hippuric acid. 



Cystine. 



The amino acid, cystine, is present in greatest amount in some of 

 those proteins belonging to the group of sclero-proteins, namely, the 

 keratins. It has been found in the liver and other organs and occasion- 

 ally forms concretions in the bladder and deposits in the urine 

 (cystinuria) . The amount excreted in these conditions is small, but 

 from 0-5-1 gm. have been recorded per diem. 



Preparation. 



Cystine is prepared by the hydrolysis of keratins. Folin has 

 shown that cystine can be readily obtained by the hydrolysis of wool. 



Wool is hydrolysed into its constituent amino acids by boiling 

 it under a reflux condenser with concentrated hydrochloric acid for 

 about five hours in the proportion of 50 gm. wool to 100 c.c. acid. 



Solid sodium acetate is slowly added to the hot solution until 

 the reaction of the solution is no longer acid to congo red. A dark 

 precipitate containing the cystine comes down on cooling. The 

 precipitate is filtered ofT when the solution is cold and washed with 

 cold water. It is dissolved in 5 per 

 cent, hydrochloric acid, filtered from 

 tarry matter and the solution is 

 boiled with animal charcoal till it is 

 colourless. The solution is filtered 

 whilst hot and hot sodium acetate 

 solution added until it is neutral 

 to congo red. Cystine crystallises 

 out in the typical hexagonal plates 

 on cooling. 



Properties. 



Cystine crystallises in colour- 

 less hexagonal plates or prisms (Fig. 

 31). It is almost insoluble in water, 



FIG. 31. Cystine. (After Funke.) 



alcohol and ether. It dissolves in dilute acids and in ammonia and 



