Allosteric binding of K⁺ to crown ether macrocycles appended to a lanthanum double decker system

A benzocrown-appended lanthanum(III) porphyrin double decker has been synthesized which is capable of binding simple K⁺ in a cooperative, allosteric fashion to form a 1:4 complex. Analysis of this system by means of the Hill equation gave an overall binding constant (K_b) of 1.0 × 10¹⁴ (mol dm^-3)^-4 and a Hill coefficient of 4.0. This same system, however, binds Na⁺ in a conventional, linear fashion. These results indicate that the novel allosteric binding occurs as the result of 1:2 sandwich complexes between K⁺ and appended crown ether moieties of the two joined porphyrins. The suitability of lanthanum(III) over cerium(IV) for preparing this highly crowded double decker porphyrin is also noted.