Tryptophan diazotization

Diazotized arsanilic acid reacts with tyrosine and with histidine, lysine, tryptophan and arginine ... 

Recently Spande and Glenner (376) reinvestigated the reaction of 3-substituted indole derivatives with diazonium salts. In earlier work, it was proposed that the reaction of diazotized sulfanilic acid with tryptophan takes place at the 3-position. On the other hand, reaction of skatole with diazotized arsanilic acid was assumed to give a 2-substituted derivative, solely by analogy to the pyrroles (259, 395). 

Conversion of tryptophan or other indole derivatives into a nitro-samine by treatment with nitrous acid followed by diazotization with N-l-naphthylethylene diamine dihydrochloride has been recently studied by Goswami (752). The method seems to be sensitive and to follow Beer s law over of wide range of concentrations of tryptophan. Selective nitrosation of tryptophan is achieved at room temperature in 1 M HCl and diazotization at 10° C or below. Sodium chloride was found to interfere with the assay. The temperature must be controlled with care, since the diazotization reaction is exothermic and diazonium compounds are stable only at low temperatures. This fact would explain the optimum temperature of 10 or 6° C in the diazotization reaction. 

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