Probing polyvalency in artificial systems exhibiting molecular recognition.

An approach to the study of polyvalency-the interaction of polyvalent receptors with polyvalent ligands-in unnatural systems is outlined. In this study, the complexation of dibenzylammonium cations by dibenzo[24]crown-8 or benzometaphenylene[25]crown-8 is utilized as the component receptor-ligand interaction. Two analogous multivalent receptors-each containing either seven dibenzo[24]crown-8 (DB24C8 CLUSTER) or seven benzometaphenylene[25]crown-8 (BMP25C8 CLUSTER) moieties appended to a modified beta-cyclodextrin core-were prepared in moderate yields. For each of these multivalent receptors, complementary mono- and divalent ligands containing one or two dialkylammonium centers, respectively, were prepared in good yields. These ligands contained fluorine atom substituents to allow their interactions with crown ether compounds to be probed by (19)F NMR spectroscopy. The complexation of these monovalent ligands with the DB24C8 CLUSTER and the BMP25C8 CLUSTER was studied by determining the average binding constant (K(AVE)) between the receptors and ligands. The abilities of the crown ether clusters to complex with these monovalent ligands was compared with those of the monovalent crown ethers dibenzo[24]crown-8 and benzometaphenylene[25]crown-8. In both instances, it was found that clustering seven crown ethers together into one molecule is detrimental to the abilities of the crown ether moieties to complex with monovalent dialkylammonium ligands. The complexation of the divalent ligands by the DB24C8 CLUSTER and the BMP25C8 CLUSTER was then studied-again by determining K(AVE)-and their abilities to complex with these ligands was compared with those of their respective component interactions. By determining K(AVE) for the polyvalent interaction, it was possible to calculate an association constant, K(POLY), for the binding of the divalent ligands by the DB24C8 CLUSTER and the BMP25C8 CLUSTER compounds. In both instances K(POLY) for the polyvalent interaction was found to be approximately 2 orders of magnitude higher than the association constants, K(A), for the component interaction.