1.D.143. The Hexameric Cyclic Helicate/(Fe(II))5-coordinated Pentafoil Knot (CHPK) Anion Transporter Family
An (Fe(II))6-coordinated triply interlocked ('Star of David') catenane link) (Leigh et al. 2014) and a Fe(II))5-coordinated pentafoil knot selectively transport anions across phospholipid bilayers (August et al. 2020). Allostery, topology, and building block stoichiometry all play important roles in the efficacy of the ionophoric activity. Multiple Fe(II) cation coordination by the interlocked molecules is crucial: the demetalated catenane exhibits no anion binding in solution, nor any transmembrane ion transport properties. However, the topologically trivial, Lehn-type cyclic hexameric Fe(II) helicates-which have similar anion binding affinities to the metalated Star of David catenane in solution-also display no ion transport properties. The unanticipated difference in behavior between the open- and closed-loop structures may arise from conformational restrictions in the linking groups that likely enhances the rigidity of the channel-forming topologically complex molecules. The (Fe(II))6-coordinated Star of David catenane, derived from a hexameric cyclic helicate, is 2 orders of magnitude more potent in terms of ion transport than the (Fe(II))5-coordinated pentafoil knot, derived from a cyclic pentamer of the same building block. The reduced efficacy is reminiscent of multisubunit protein ion channels assembled with incorrect monomer stoichiometries (August et al. 2020).
A metal-organic framework featuring fused Star-of-David catenanes with two triangular metallacycles with opposite handedness are triply intertwined forming a Star-of-David catenane. Each catenane fuses with its six neighbors to generate a porous twofold intercatenated gyroid framework. The compound possesses exceptional stability and exhibits multiple functionalities including highly selective CO2 capture, high proton conductivity, and coexistence of slow magnetic relaxation and long-range ordering (Huang et al. 2017).