engleski

Structure-activity correlations for ring-alkylated derivatives of indole-3-acetic acid

As a part of molecular recognition studies on auxins, the growth-promoting activities of ring-alkylated (alkyl = 4-, 5-, 6-, or 7-methyl, 4- or 5-ethyl, 5-n-propyl, or 5-n-butyl) indole-3-acetic acids in the Avena coleoptile straight-growth assay were determined and correlated with structural parameters. While half-optimal concentrations were between 4.5 and 6.0 micromol/L for all compounds examined, the elongation response at the optimal concentration decreased notably as the length of the alkyl chain increased. The configurations of the auxin molecules were determined by X-ray diffraction analysis in the crystalline state and are unlikely to change in solution, or at the active site of an auxin receptor. The conformations of the CH_2COOH moiety (hypothesized to be of critical importance for biological activity) and of the higher-alkyl chains, in a physiological environment, cannot be predicted from crystallographic data. Molecular mechanics and dynamics simulations reveal, however, that the energy barriers between the individual conformers do not exceed a level of several kcal/mol, and are thus of the same order of magnitude as weak intermolecular interac! tions, such as van der Waals forces and hydrogen bonding. (Supported in part by Croatian Ministry of Science and Technology research contracts 00980608 and 00980707, and by US-Croatian Research Agreement JF202.)

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