GLYOXALINE OR IMINAZOLE DERIVATIVES 285 



being kept slightly alkaline with sodium carbonate. The precipitate is filtered 

 off, washed and dissolved in a minimum of dilute hydrochloric acid. The 

 filtered solution is considerably diluted, some mercuric chloride added and 

 precipitated with sodium carbonate. The precipitate is filtered off, washed, 

 suspended in water and decomposed with hydrogen sulphide. The filtrate 

 from mercuric sulphide on evaporation yields histidine monochloride. 10 

 litres of blood give from 70-90 gm. (Knoop, 1907). 



Properties. 



Histidine forms small platelets which melt at 253 with decomposition. 

 It is easily soluble in water, very slightly soluble in alcohol and insoluble in 

 ether. 



Its solution is alkaline in reaction ; it forms salts with acids and also 

 double salts with gold chloride, etc. 



It is precipitated by silver nitrate and ammonia or baryta water, by mer- 

 curic sulphate in sulphuric acid solution and by phosphotungistic acid. 



Reactions. 



(1) A solution of histidine treated with sodium hydroxide and a trace 

 of copper sulphate and heated gives a violet colour, which changes to red 

 (compare biuret reaction). 



(2) A solution of histidine made alkaline with sodium carbonate and 

 treated with 3-5 c.c. of a fresh alkaline (Na 2 CO 3 ) solution of about -05 gm. 

 of diazobenzene sulphonic acid l gives on standing a deep cherry-red colour, 

 which changes to orange on acidifying (Pauly). 



The delicacy of this reaction is i in 20,000 ; with i in 100,000 a pale red 

 colour is produced. 



Tyrosine gives a similar colour with the reagent and must consequently 

 be absent from the solution. 



A modification of the reaction by which histidine can be distinguished 

 from tyrosine was devised by Totani. 2 



(3) An acid aqueous solution of histidine is treated with bromine water 

 until it has a permanent yellow colour. On heating, the yellow colour dis- 

 appears, but the solution becomes gradually red until it assumes a deep wine- 

 red colour and a black amorphous precipitate settles out. This reaction is 

 positive in a dilution of i in 1000. 



Pilocarpine. 



This basic substance or alkaloid (p. 351) contains an iminazole 

 ring. Its formula is probably the following : 



/CH 3 



C 2 H 5 . CH CH-CH 2 C N( 



\CH. 



CO O CH 2 H . C N/* 



1 This is prepared by stirring 2 gm. of finely powdered sulphanilic acid into a paste 

 with 3 c.c. of water and 2 c.c. of cone, hydrochloric acid. The paste is treated, within a 

 period of i minute, with a fresh solution of i gm. of potassium nitrite in i to 2 c.c. of 

 water, the mixture being cooled after each addition with cold water. Most of the sulphanilic 

 acid rapidly dissolves and a thick white crystalline precipitate of diazobenzene sulphonic 

 acid forms. It is filtered off after a few minutes and washed with a little water. 



2 Biochem. J., 1015, 9, 385. 



