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Absorbance and fluorescence spectra of ring-substituted indole-3-acetic acids

Indole-3-acetic acid (IAA) is the principal auxin in plants. Ring-substituted analogues with distinctive spectroscopic properties could be useful probes for studying its interaction with proteins involved in the hormonal response. Here we explore the absorbance and fluorescence spectra of 28 such derivatives which all show auxin activity. They are characterized by a group of 2-3 absorbance maxima at 260-310 nm and a maximum of about four times their intensity at 215-230 nm. 4- and 7-fluoro-IAA show blue shifts (maximally 10 nm), most other derivatives red shifts (maximally 15 nm for dichloro-IAAs). All IAA analogues examined, with the exception of chloro-, bromo-, and 4- or 7-fluoro-derivatives, fluoresce at 345-370 nm when excited at 275-280 nm. The emission sprectra of hydroxy-, methoxy- and benzyloxy-derivatives are blue shifted with respect to unsubstituted IAA. Alkyl substituents slightly shift the emission maximum to higher wavelengths if inserted at position 2, but to shorter wavelenghts if located at positions 4 and 7. Quantum yields for ring-substituted IAAs in aqueous buffer, pH 5-7, are generally somewhat lower than for the parent compound (0.3), except for 6-fluoro-IAA (0.5).

auxin ; ring-substituted indole-3-acetic acid ; uv absorbance ; fluorescence

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