84 PROPERTIES OF CERTAIN AMINO-ACIDS. [CH. IV. 



84. Dissolve a small amount of cystine in one or two cc. of 5 

 per cent, sodium hydroxide. Add a drop or two of lead acetate and 

 boil for a minute. The solution is darkened owing to the formation 

 of lead sulphide (see Ex. 25). 



Histidine. HC 



N 



C. CH 2 CH COOH. 



NH 



CH 



85. The preparation of histidine mono-hydrochloride. 



(i.) Place i litre of defibrinated ox or sheep blood (or preferably 

 I litre of the centrifuged corpuscular mass from blood) 

 into a 2-litre, round-bottomed flask. 



(ii.) Add 500 cc. of pure concentrated hydrochloric acid, shaking 

 well during the addition. 



(iii.) Heat on a boiling water bath, shaking at intervals, for 2 

 to 3 hours, until the precipitated blood proteins have re- 

 dissolved. 



(iv.) Boil on a sand bath under a reflux condenser for 

 10 hours. 



(v.) Transfer to an evaporating basin and remove the greater 

 part of the hydrochloric acid by evaporating on a boiling 

 water bath in a flue chamber. 



(vi.) Add 40 per cent, caustic soda until the mixture is only 

 slightly acid to litmus paper. 



(vii.) Allow to stand over-night, and filter on the pump. Wash 

 the precipitate with hot water, adding the washings to 

 the main filtrate. 



