engleski

Structural studies on 5-(n-alkyl)-substituted derivatives of the plant hormone, indol-3-ylacetic acid

The biological properties of the plant hormone (auxin) indol-3-ylacetic acid (IAA) and its ring-substituted derivatives have so far been rationalized by hypotheses based on incomplete structural data. Here we compare the molecular structures of IAA and 5-(n-alkyl)indol-3-ylacetic acids (alkyl = methyl, ethyl, n-propyl, n-butyl), as revealed by X-ray crystallography and molecular mechanics calculations, and their growth-promoting activities in the Avena mesocotyl test. For the compounds studied, the 5-(n-alkyl) residue was in an extended zig-zag conformation with the C-alpha— C-beta bond perpendicular to, and the C-beta— C-gamma bond pointing away from, the aromatic nucleus. In the crystalline state, the CH2-carboxyl bond was perpendicular to the indole ring plane for 5-methylindol-3-ylacetic acid, and approximately in that plane for the other IAA derivatives studied. Crystal packing influences appear to be responsible for this effect, as indicated by the results of molecular mechanics calculations. In the potential energy-minimized conformations for the isolated molecules in the gas phase, perpendicular orientation for the carboxyl group was always predominant. The plant growth-promoting properties of the compounds studied were not dramatically different. This indicates that a 5-substituent as bulky as n-butyl, in the orientation detailed above, does not sterically obstruct interaction with the auxin-binding protein(s) involved in the growth response.

auxin; ring-substituted indol-3-ylacetic acid; X-ray crystallography; molecular modeling; structure-activity correlation; Avena mesocotyl test

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