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

Abstract

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][2][3][4][5][6][7][8][9][10][11][12] have been reported because of the lipophilic nature of porphyrins. In a previous work, 8) we reported amino acid chiral recognition by a water-soluble zinc porphyrin, where three interactions act as the recognition factors. Two of the interactions are attractive, but the remaining interaction has not yet been identified as attractive or repulsive. Here, we examined the binding behavior of zinc porphyrins 1-4 with amine guests in basic aqueous solutions, where the porphyrin substituents near the central zinc ion can differently interact with the bound guests. The binding data suggested the existence of three types of simultaneous attractive interactions between the zinc porphyrins (2, 4) and the anions of nicotinic acid and 3,5-dicarboxypyridine. Results and DiscussionIn the absence of amine, zinc porphyrins exist as five coordinated species with a bound H 2 O molecule in aqueous solution.13) The N atom of amines is not protonated under the basic experimental conditions (pH 10.4), thereby the N atom of the amines can coordinate to the zinc ion and substitute the bound H 2 O of the porphyrins. The binding equilibrium between zinc porphyrin ZnP and amine L is expressed as follows:where the binding constant is given as Figure 1 shows visible spectra of 4 upon titration of nico. The spectral changes correspond to the N coordination of the 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 p p system or carboxyl group. This observation suggested that interligand attractions of Coulomb and CH-p p 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.

Water-Soluble Zinc Porphyrins as Artificial Receptors for Amino Acids

“…3,17) Proton NMR spectra in D 2 O were obtained from a JEOL JNM-GSX-400 (400 MHz) spectrometer with sodium 3-trimethylsilyl-propanesulfonate as the internal standard. Visible absorption spectra were recorded on a Hitachi U-3000 spectrophotometer.…”