engleski

The electronic structure of auxin molecules correlates with their biological activity

As early as 1963, Hansch et al. (J. Am. Chem. Soc. 85, 2817) successfully correlated the auxin activity of ring-substituted phenoxyacetic acids to their lipophilicity and the electronic properties of the substituents, as expressed by Hammett's sigma-constants. Porter and Thimann (Phytochemistry 4, 229, 1965) applied this approach to ring-substituted indole-3-acetic acids (IAAs), introducing the concept of a "fractional positive charge" at the indole nitrogen, a parameter which cannot be defined on an absolute scale. This was unfortunate, as practical and theoretical indole chemistry agree in attributing a negative charge to the indole nitrogen. As Thimann's "charge-separation theory" was nevertheless based on accurate experimental data we reexamined his conclusions using a larger set of ring-substituted IAAs. The electron density at the indole nitrogen was estimated from N-H dissociation constants in strongly alkaline medium and from H-1 NMR chemical shifts in chloroform and methyl sulfoxide solution. Ring-alkylated IAAs with about the same or higher electron density at the indole nitrogen than in the unsubstituted parent compound had about the same or weaker growth-promoting activity in stem elongation bioassays. In contrast, ring-halogenated analogues with reduced electron density at the indole nitrogen tend to be more active. Simple one-parameter correlations were not expected because factors such as lipophilicity and the geometry of the active site of the auxin-binding protein(s) involved in the growth response also require attention. Importantly, however, auxin binding has been postulated to include interaction of the indole nucleus with a "flat platform" represented by the residue of at least one aromatic amino acid. Our results strongly indicate that this interaction involves not only hydrophobic bonding, but also electronic factors, such as the formation of charge-transfer complexes.

auxin; structure-activity relations; ring-substituted indole-3-acetic acid; IAA; NMR spectroscopy; NH dissociation constant; electron density

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