72 



THE CHEMICAL CONSTITUENTS OF THE BODY 



13. In many proteids the presence of a hexone base, described as histidin, 

 C 6 H 9 N 3 O 7 has been demonstrated. It appears to be a pyrimidin derivative 



NH CH a 

 HC CH 

 N C.CH(NH a ).COOH. 



In the following table are brought together after Ehrstrom the most impor- 

 tant nuclei occurring in the proteid molecule, arranged according to the number 

 of carbon atoms which they contain. 



Certain of these groups can be recognized by characteristic color reactions. 

 (1) The xanthroprote'ic reaction gives a yellow color with strong nitric acid; 

 after neutralization with ammonia or a caustic alkali, the color passes over to 

 orange or reddish brown. The reaction depends upon the presence of the ben- 

 zol ring in the proteid molecule (phenylalanin, tyrosin, indol). (2) Millon's reac- 

 tion gives a red coloration to the precipitate or to the fluid, when a solution 

 of mercuric nitrate containing some nitrous acid is added to a solid proteid or 

 to a proteid solution. This indicates the presence of the oxyphenyl group 



