Indoles and auxins : XIII. Identification and analysis of naturally occurring indoles via electron donor-acceptor complexes

Abstract

The formation of electron donor-acceptor complexes of some representative indoles with a number of electron acceptors has been investigated with particular emphasis on qualitative and quantitative aspects of analysis involving thin-layer chromatography (TLC). This non-destructive method shows a detection limit for indoles, after chromatography, from 0.5 to 1 μg, depending on the structure of the indole and the electron acceptor employed. An in situ method was developed for quantitation of indoles on the sprayed chromatograms and a linear relationship between concentration of the indole and detector response of a TLC scanner for reflectance at the charge transfer band maxima was observed in the range of 0.5–10 μg. With the same electron acceptor, charge transfer band maxima from complexes on a layer of silica gel shift to longer wavelength when substituents on the indoles are changed from electron withdrawing ( e.g. −CN) to electron donating ( e.g. −OH). Separate mass spectra for indoles and complexing reagents can be obtained directly from the complex, as previously observed, at different temperatures. Some of the newer electron acceptors having low vapor pressures allow application of this method at much higher temperatures, i.e. less volatile indoles can be identified in this manner.