Structural characterization and auxin properties of dichlorinated indole-3-acetic acids

The dichlorinated indole-3-acetic acids: 4,5-Cl₂-IAA, 4,6-Cl₂-IAA, 4,7-Cl₂-IAA, 5,6-Cl₂-IAA, 5,7-Cl₂-IAA and 6,7-Cl₂-IAA were synthesized and characterized by X-ray structure analysis to unambiguously identify the substances for bioassays required to establish structure activity relationships of auxins and their analogues. Straight-growth tests were performed on Avena sativa coleoptiles to correlate their auxin activity with molecular properties which could reveal information on the topology of the auxin binding site. Structure/activity correlations revealed that the 5,6-Cl₂-IAA molecule, by virtue of its size and shape, fits particularly well into the active site cavity of the receptor protein. The main contact of the substrate or inhibitor in the receptor active site via the carboxylic group determines their orientation in the active site cavity. As a consequence, the 5,6-substituted sites protrude into the widest part of the active site whereas the 7-, 4-, and 5-substituted sites are oriented towards the narrowest part of the active site. These topological parameters are in agreement with the high auxin activity of 5,6-Cl₂-IAA and the low activity of 4,7-Cl₂-IAA.