PROTEINS 77 



cent was obtained by Abderhalden from globin, the protein constituent 

 of oxyhemoglobin, and about 13 per cent by Kossel and Kutscher from 

 the protamine sturine. 



Crystals of histidine dichloride are shown in Fig. 27. 



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

 nent. If the tube be heated, 1 the color will disappear and will shortly be re- 

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

 Usually black, amorphous particles separate out and the solution becomes 

 turbid. 



The reaction cannot be obtained in solutions containing free alkali. 

 It is best to use such an amount of bromine as will produce a permanent 



V - .; : ; . 



FIG. 27. HISTIDINE DICHLORIDE. 



yellow color in the cold. The use of a less amount of bromine than this 

 produces a weak coloration, whereas an excess of bromine prevents the 

 reaction. The test is hot very delicate, but a characteristic reaction 

 may always be obtained in i : 1000 solutions. The only histidine de- 

 rivative which yields a similar coloration is imidazolylethylamine, and 

 the reaction in this case is rather weak as compared with the color ob- 

 tained with histidine or histidine salts. 



Valine, C 5 HnNO 2 . The ammo-valeric acid obtained from proteins 

 is a-amino-isovaleric acid, and as such bears the following formula: 



CH 3 NH 2 



I I 



H C C COOH. 



I I 

 CH 3 H 



1 The same reaction will take place in the cold more slowly. 



