Further evidence of tautomerism in chromatograms of indolyl-3-pyruvic acid

Abstract

A freshly prepared solution of indolyl-3-pyruvic acid treated with one equivalent of NaHCO3 soon begins to decompose. Paper chromatograms of samples drawn at regular intervals show the fading of the original spot (spot 2) and a corresponding appearance of 3 other spots (3, 4 and 5). The most conspicuous of these, spot 4, first increases and, after 20 min., decreases and fades out. Good separation of spots 2 and 4 is achieved only when precautions are taken against decomposition during chromatography. With a slow solvent like propan-2-ol, in alkaline medium, a number of decomposition products crowd the chromatogram and only a single spot with the characters of spots 2 and 4 can be identified. Color tests for indolyl-3-pyruvic acid (Gordon-Weber, reagent, Ehrlich''s reagent, dichlorophenolindophenol, FeCl3, ammoniacal AgNO3, dinitrophenylhydrazine and Van Eck''s reagent) applied to chromatograms show that only spots 2 and 4 respond to all of them, suggesting that spots 2 and 4 represent two forms of indolyl-3-pyruvic acid. The relative intensity of spots 2 and 4 was shown to depend on pH, temperature, aging and quality of solvent, i.e., factors influencing tautomerization. It is therefore suggested that spots 2 and 4 correspond to the enol and keto tautomers of indolyl-3-pyruvic acid respectively, and that the tail in between is due to tautomerization during chromatography.