Remarkably Large Positive and Negative Allosteric Effects on Ion Recognition by the Formation of a Novel Helical Pseudocryptand

Abstract

In supramolecular chemistry, a great deal of attention has focused on regulating guest binding via an external stimulus. To utilize the same effector for both highly guest-selective positive and negative allosteric effects, however, stricter and more precise regulation of the host structure is required. A novel allosteric host 1 binds Fe(II) to afford the pseudocryptand,1·Fe(II), which bears a cavity that is surrounded by three polyether chains in a helical fashion. The binding selectivity of 1 (Na⁺ > K⁺ > Rb⁺ > Cs⁺) is the opposite of 1·Fe(II) (Cs⁺ > Rb⁺ > K⁺ > Na⁺). Single-ion transport through a liquid membrane shows ion selectivity similar to the equilibrium constants. To the best of our knowledge, this is the first example of an allosteric recognition system, in which the same effector, that is, Fe(II), exhibits both large positive and negative allosteric effects on equilibrium and dynamic recognition events. The X-ray analysis and ¹H NMR examination indicate that the combination of the macrobicyclic effect and the intramolecular interchain interactions (CH−π interaction and steric hindrance) finely controls the positive and negative allosteric effects, which depend on the size of the guest. The helical framework opens a new general method for constructing more sophisticated, controllable receptors for helical biomolecules, for example, DNA and proteins, and helical molecular devices such as a molecular coil or spring responding to a stimulus.