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Title: STRUCTURAL STUDIES ON MONOHALOGENATED DERIVATIVES OF THE PHYTOHORMONE INDOLE-3-ACETIC ACID (AUXIN)

Author
item NIGOVIC, BILJANA - RUDER BOSKOVIC INSTITUTE
item KOJIC-PRODIC, BISERKA - RUDER BOSKOVIC INSTITUTE
item ANTOLIC, SNJEZANA - RUDER BOSKOVIC INSTITUTE
item TOMIC, SANJA - RUDER BOSKOVIC INSTITUTE
item PUNTAREC, VITIMIR - RUDER BOSKOVIC INSTITUTE
item Cohen, Jerry

Submitted to: Acta Crystallographica Section B: Structural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/22/1995
Publication Date: N/A
Citation: N/A

Interpretive Summary: Indole-3-acetic acid (IAA) is an important plant hormone that is found in all plants and in all plant tissues. In some plants, such as peas and faba beans, a chlorine containing form of IAA (4-Cl-IAA) is also found and thought to be specifically involved in fruit (seed and pod) development. Food industry literature noted the existense of this compound because it was found in canned peas in an analysis designed to detect chlorine containing pesticides. While 4-Cl-IAA is naturally found in such plant materials, we know very little about this compound and do not know why it is in peas and faba beans yet absent in most other plants. It is known, however, that 4-Cl-IAA shows very strong plant hormone activity (10 times that of IAA) when applied to plants. In order to understand this compound better, we studied the molecular structure of 4-CI-IAA and several related compounds and compared them to that of IAA. These data will be useful to hetercyclic chemists, to plant biochemists, physiologists and molecular biologists interested in hormone activity, and may find application in the biorational design of growth regulating compounds. These data may also prove important to food chemists and nutritionists interested in studies on the fate of this unique compound in human and animal diets.

Technical Abstract: The physiological properties of the phytohormone (auxin) indole-3-acetic acid (IAA) and its ring substituted derivatives have so far been rationalized by a number of contradictory hypotheses based on incomplete structural data deduced mainly by inspection of molecular models. In order to give more evidence for structure-activity relationships of monohalogenated IAAs the molecular structures of the natural auxin 4-CI-IA as well as 5-CI-IAA, 6-CI-IAA, 7-CI-IAA, and 5-Br-IAA have been compared as revealed by X-ray analysis, molecular mechanics and dynamics. The influence of the substitution site of the halogen atom and bioactivity is discussed. The typical structural features of molecules studied are the slight distortion of part of the indole nucleus around C7: bond length C6-C7=1.368(6)A and C6-C7-C71=117.6(3) (average values of five structures and seven molecules). The conformations of monohalogenated indole-3-acetic cacid molecules, characteristic ones for auxins, are defined by rotations about two bonds only: one describes the relative orientation of a side chain towards the indole moiety and the second one the orientation of the carboxylic group. The results of X-ray structure analysis, molecular mechanics and dynamics revealed the folded shape of the molecules in all compounds studied. Ab initio calculations showed that the planar conformation can be adopted as well.