Binding Enhancements of Nicotinic Acid to Water-Soluble Zinc Porphyrins Based on Triple Attractions of Coordination, Coulomb, and CH-π Interactions

The binding of amines to artificial zinc-porphyrin receptors 1-4 was examined in basic aqueous solutions. For nicotinic acid and 3,5-dicarboxypyridine, substantial binding enhancements were observed compared to other amines with no π system or carboxyl group. This observation suggested that interligand attractions of Coulomb and CH-π interactions in addition to N-atom coordination can act effectively as recognition factors. The differences in the Coulomb interaction between carboxylate and sulfonate anions were also discussed. The examination of the binding of small biomolecules with artificial receptors in aqueous media is one of the traditional approaches to obtain fundamental information on molecular recognition in biological and pharmaceutical conditions. Binding enhancements and selectivity for guests by receptors result from additively and/or cooperatively combined weak non-covalent interactions. Synthetic metalloporphyrins have been widely used as receptors for biomolecules and their analogs； however, not many studies in aqueous solutions 1-12) have been reported because of the lipophilic nature of porphyrins.