Molecular recognition blueprint: Isolated conformers of Indole-3-acetic acid predispose the plant hormone for receptor binding

We report the gas-phase conformational study of 3-indoleacetic acid (IAA), a key plant hormone, using rotational spectroscopy combined with quantum chemical calculations. Of the five theoretically predicted structures at the B3LYP-D3BJ/6-311++G(d,p) level, we experimentally identified four conformers. The conformers differ mainly in the orientation of the carboxylic acid group and are stabilized by different non-covalent interactions. Notably, the conformers with the carboxylic group out of the indole ring plane closely resemble the bioactive structure found in the TIR1 ubiquitin ligase receptor complex, the PIN3 transporter, the AdmX transcriptional activator, and the auxin importer AUX1. This work provides fundamental insights into the intrinsic structural preferences of this crucial auxin molecule.