74 PHYSIOLOGICAL CHEMISTRY. 



Histidine, C 6 H 9 N 3 O 2 . Histidine is a-amino-ft-imidazol-pro- 

 pionic acid with the following structural formula : 



H NH 



HC C A A 



JJLV_y \j \j w 



I I U 



HN\/N 



COOH. 



CH 



The histidine obtained from proteins is laevo-rotatory. It has 

 been obtained from all the proteins thus far examined, the majority 

 of them yielding about 2.5 per cent of the amino acid. HOW- 

 FIG. 25. 



v ., jjp^ ' 



HISTIDINE BICHLORIDE. 



ever, about 1 1 per cent was obtained by Abderhalden from globin, 

 the protein constituent of oxy haemoglobin and about 13 per cent 

 by Kossel and Kutscher from the protamine sturine. 



Crystals of histidine dichloride are shown in Fig. 25, above. 



Knoop's Color Reaction for Histidine. To an aqueous solu- 

 tion of histidine or a histidine salt in a test-tube add a little bromine 

 water. A yellow coloration develops in the cold and upon further 

 addition of bromine water becomes permanent. If the tube be 

 heated, 1 the color will disappear and will shortly be replaced by a 

 faint red coloration which gradually passes into a deep wine red. 

 Usually black, amorphous particles separate out and the solution 

 becomes turbid. 



same reaction will take place in the cold more slowly. 



