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 (Kb) 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.
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