Submitted to: Journal of Plant Growth Regulation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/3/1998
Publication Date: N/A
Interpretive Summary: Indole-3-acetic acid (IAA) is an important plant hormone that is found is all plants and in all plant tissues. In some plants, such as fresh and canned peas and in faba beans, a chlorine containing form of IAA has also been found. Synthesis of substituted compounds in which specific hydrogens are changed to a halogen like chlorine has been used in medicinal chemistry to alter the metabolic stability of target compounds without major effect on the chemical properties. Such studies have proven useful for understanding the interaction between a particular compound and its biological receptor. In order to understand the metabolism and activity of IAA and related compounds better, we studied the structure and biological activity for substituted IAA's with two chlorines. These data will be useful to organic chemists, biochemists and chemical engineers interested in the relationship between activity and structure and may find application in the biorational design of growth regulating compounds and certain pharmaceuticals. These data may also prove important to food chemists and nutritionists interested in studies on the fate of halogenated IAA and its metabolites in human and animal diets.
Technical Abstract: 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 coleoptile to correlated their auxin activity with molecular properties which could reveal information on the topology of the auxin binding site. Structure/activity correlations reveled that the 5,6-Cl2-IAA molecule, by virtue of its size and shape, fits particularly well into the active cavity of the receptor protein. The main contact of the substance 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- substitutes sites are orientated towards the narrowest part of the active site. These topological parameters are in agreement with the high auxin activity of the 5,6-Cl2-IAA and the low activity of the 4,7-Cl2-IAA.