ARTIFICIAL ALLOSTERIC IONOPHORES - REGULATION OF ION RECOGNITION OF POLYETHERS BEARING BIPYRIDINE MOIETIES BY COPPER(I)

Artificial allosteric ionophores, oligoethylene glycols bearing 2,2-bipyridine derivatives at the termini, were synthesized. Solvent extraction experiment indicated that the polyethers bind Cu+ firmly and selectively among heavy metal ions because of a chelate and a steric effect of the two bipyridines on the ligation. The complexation with Cu+ takes place rapidly and quantitatively in organic media to result in intramolecular cyclization of the linear polyether moiety, giving a novel type of crown ethers, pseudocrown ethers. The UV-vis and H-1 NMR spectroscopies indicated that the bipyridine complexes have a tetrahedral geometry. Allosteric regulation of ion recognition (heterotropic cooperativity) has been performed successfully in transport experiment through a liquid membrane (methylene chloride) by using the ionophore and Cu+ as an effector. The transport selectivity to alkali metal ions was dramatically enhanced by the addition of Cu+ due to the formation of the cyclic framework. The transport experiment and measurement of uptake and release rates of ionophores for alkali metals suggested that the remarkably high transport selectivity results from a suitable cavity size of the pseudocrown ring and electrostatic repulsion between alkali metal and Cu+ ions bound in the same ionophore. Moreover the pseudocrowns exhibit molecular chirality at -28-degrees-C in the presence of excess Pirkle's reagent, but racemization occurs at room temperature. In contrast, a pseudocrown formed from Cu+ and a polyether with four bipyridines maintains its molecular chirality even at room temperature, promising application of the pseudocrowns to asymmetric recognition. The facts obtained here indicate clearly that our strategy, conversion of a linear polyether with heavy metal binding sites to the corresponding pseudocrown by the addition of heavy metals, is quite convenient and powerful for modulation of molecular recognition and molecular information.