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Comparison of the structures of the plant growth hormone indole-3-acetic acid, and six of its amino-acid conjugates

The crystal structures of six biologically active cnojugates of the plant growth hormone (auxin), indole-3-acetic acid (IAA), with the amino acids L-alanine (1), alpha-amino-L-butyric acid (2), L-norvaline (3), DL-aspartic acid (4), L-isoleucine (5), and delta-aminovaleric acid (6) were determined. In these conjugates the conformations of the indol-3-ylacetyl moieties are very similar to that observed in free IAA, as are the values of bond lengths and intramolecular contact distances within the IAA moiety. The indole ring system and the C atom of the adjacent methylene group are coplanar, whereas the -COOH or -CONR residues, respectively, adopt a folded conformation. The carbonyl group of the free hormone points towards the indole ring ; however, in the amino-acid conjugates it points away from the ring system. The orientation of the amino-acid side chains with respect to the aromatic ring varies in compounds (1) to (6). Consistently, however, only the region of the IAA moiety in immediate proximity to the -CO group is sterically blocked by the conjugant. The rest of the indole nucleus, which appears to include the -NH group, remains potentially available for binding competitively (with free IAA) to proteins such as auxin receptors and enzymes regulating intracellular levels of growth hormones.

plant hormone ; auxin ; indole-3-acetic acid ; IAA ; amino acid ; conjugate ; X-ray analysis ; molecular structure ; structure-activity correlations

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