engleski

Structure-Activity Correlations for Ring-Fluorinated Indole-3-Acetic Acids

As a part of molecular recognition studies on auxins, a series of indole-3-acetic acids (IAAs) fluorinated at ring positions 4, 5, 6, or 7 was examined. The substituent profoundly influences the electron distribution within the aromatic ring system, but does not require much more space than the hydrogen atom it replaces (van der Waals radii: H, 1.20 &Aring ; ; F, 1.35 &Aring ; ). Indeed, X-ray structure analysis and molecular mechanics calculations confirmed that the conformations of all the fluoro-IAAs studied are essentially the same as for the unsubstituted parent compound, with the side chain CH_2-COOH bond perpendicular to the ring plane (folded conformation). According to the results of molecular dynamics simulations, this remains an energetically preferred, highly populated conformation in aqueous solution, although there is considerable rotation about all single bonds in the side chain. The dose response curves for 5-F-IAA, 6-F-IAA, and 7-F-IAA in the Avena coleoptile straight-growth test were essentially superimposable to that for unsubstituted IAA, with a flat maximum at 10 micromol/L (range of concentrations examined: 1 to 100 micromol/L). In contrast to these results, the optimal concentration for 4-F-IAA was 1 micromol/L, and the optimal growth response was about 40% larger than for the other compounds tested.

auxin; indole-3-acetic acid; IAA; fluoroindole-3-acetic acid; molecular recognition; X-ray diffraction; molecular mechanics; molecular dynamics; structure-activity correlation

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