engleski

Structural studies on 5-(n-alkyl)-substituted derivatives of the plant hormone indole-3-acetic acid

The biological properties of the plant hormone (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. To permit a more detailed insight into structure-activity correlations, we here compare the structures of IAA and 5-(n-alkyl)indole-3-acetic acids (alkyl = methyl, ethyl, propyl, butyl), as revealed by X-ray crystallography and molecular mechanics calculations, and their growth-promoting activity in a standard bioassay, the Avena mesocotyl test. The following molecular properties were identical for the compounds studied: The benzene part of the indole nucleus was slightly distorted around C7, as previously observed for many other indole derivatives (statistical data included). The 5-(n-alkyl) residue was in an extended zigzag conformation with the C_alpha-C_beta bond perpendicular to, and the C_beta-C_gamma bond pointing away from, the aromatic nucleus. Conformational differences were observed with respect to the orientation of the CH_2-carboxyl bond in the crystalline state: perpendicular to the indole ring plane for 5-methylindole-3-acetic acid (as previously found for IAA), 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. The potential-energy-minimized conformations in the crystal lattice environment corresponded closely to those observed by X-ray analysis while, for the isolated molecules in the gas phase, perpendicular orientation of the carboxyl group was always predominant. The plant-growth-promoting properties of the compounds studied were not dramatically different. In particular, 5-methyl- and 5-(n-butyl)indole-3-acetic acids showed very similar activity. 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 ; indole-3-acetic acid ; IAA ; 5-alkylindole-3-acetic acid ; X-ray crystallography ; molecular modeling ; biological activity ; structure-activity correlations

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