Organic Substances 121 



naphthol, respectively, differ from one another sharply in 

 color. The reaction is highly specific; under conditions of 

 moderate alkalinity all phenols except those substituted in 

 the para and both ortho positions will couple; the only other 

 substances found in the tissues which will give a similar reac- 

 tion are heterocyclic compounds (histidine; probably also 

 proline ) . According to Pauly J tryptophane does not couple; 

 according to Danielli,^ it does. In the writer's experiments no 

 color was produced with tryptophane with five different dia- 

 zonium salts. In general, the color of azo dyes resulting from 

 amino acids is yellowish or pale orange-brownish. 



2) The indophenol reaction.— This consists in the forma- 

 tion of bluish indophenol dyes when phenols unsubstituted 

 in the para position are oxidized in the presence of aromatic 

 amines. The reaction can be performed in a variety of ways. 

 One of the simplest is Gibbs's method,^ in which the amine 

 and the oxidant are combined into a single substance (2,6- 

 dichloro- or dibromo-quinonechloroimide ) ; it also possesses 

 the advantage of producing somewhat darker shades than the 

 other variants. 



3) The ferric chloride reaction.— Yhenoh give character- 

 istic color reactions (green, blue, purple, depending on the 

 nature of the phenol ) with a dilute solution of ferric chloride. 

 The shades produced in tissue sections are much too pale to 

 be useful. 



In addition to giving the reactions mentioned, diphenols 

 are strong reducing agents. They will reduce an ammoniacal 

 silver nitrate solution to metallic silver ("argentaffin reac- 

 tion" ).^^' ^^ By dichromates and ipdates they are oxidized to 

 quinones and other brownish, more or less insoluble, prod- 

 ucts of poorly known constitution which ultimately precipi- 



7. Pauly, H.: Ztschr. f. physiol. Chem., 42:508, 1904. 



8. Danielli, J. F.: Cold Spring Harbor Symp. Quant. Biol., 14:32, 1950. 



9. Gibbs, H. D.: J. Biol. Chem., 72:649, 1927. 



10. Cordier, R.: Bull, d'histol. appliq. a la physiol., 4:161, 1927. 



11. Hamperl, H.: Virchows Arch. f. path. Anat., 286:811, 1932. 



