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Interactions of indole derivatives with immobilized 3, 5-dinitrobenzoylphenylglycine

Aromatic interactions play an important role in ligand-receptor interactions in biological systems. The importance of such interactions in auxin recognition by the potential receptor, ABP1, is strongly suggested by a recent crystallographic study which showed that the indole moiety of the amino acid residue Trp-151 is part of the active site accommodating the synthetic auxin, naphthalene-1-acetic acid. Here we use high performance liquid chromatography to study the affinity of about 50 indole derivatives (most of them with auxin activity) to immobilized 3, 5-dinitrobenzoyl-phenylglycine which is expected to act as a pi-electron acceptor because the electron density in its aromatic ring system is significantly less than in the indole nucleus. To verify the retention mechanism, a QSPR (quantitative structure-property relationship) analysis was performed. A five-parameter model was obtained which accounted for more than 95% of the variations in the relative binding constants. The model confirmed that binding affinity is significantly influenced by the electron density in the indole nucleus. Thus, for indoles with small ring-substituents, their inductive effects predominantly determined the binding constants: electron-withdrawing substituents reduced affinity, while electron-donating substituents increased it. The presence of ring-substituents with hydrogen-bonding capacity further strengthened binding to the acceptor phase. On the other hand, bulky substituents generally decreased affinity, presumably by interfering with the interaction of the indoles with the stationary phase. The relative binding constants of dihalogenated indole-3-acetic acids were essentially determined by their substitution pattern. Indole-3-acetic acids bound more strongly than the corresponding 3-unsubstituted indoles, presumably as a result of additional hydrogen bonding and/or dipole-dipole interaction involving the carboxyl group.

indole derivatives; 3; 5-dinitrobenzoylphenylglycine; high performance liquid chromatography; quantitative structure-activity relationship

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