Abstract
A (FeII)6-coordinated triply interlocked (“Star of David”) [2]catenane (612 link) and a (FeII)5-coordinated pentafoil (51) knot are found to selectively transport anions across phospholipid bilayers. Allostery, topology, and building block stoichiometry all play important roles in the efficacy of the ionophoric activity. Multiple FeII 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 FeII 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 (FeII)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 (FeII)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.
Highlights
The orderly entanglement of molecular strands within knots[1] and links[2] can induce properties and characteristics1a2a that are beginning to be explored in areas as diverse as anion binding,[3] catalysis,[4] materials,[5] health care,[6] and the kinetic stabilization of supramolecular structures.[7]
The ion transport abilities of the metalated knot and link were compared with that of the demetalated Star of David catenane, 4, which without transition-metal coordination exhibits no anion binding in solution, and from the experimental studies of the (FeII)-coordinated Lehn-type open cyclic hexameric helicates (5 and 6), to determine the influence of (i) metal-binding allostery and (ii) molecular topology (Figure 2)
The FeII-coordinated pentafoil knot 24a and the Star of David [2]catenane 39c were prepared from ligand 1 in the presence of pores; misassembly with respect to the number of building blocks still results in a functional ion channel but with greatly different FeII salts followed by ring-closing olefin metathesis
Summary
The orderly entanglement of molecular strands within knots[1] and links[2] (catenanes) can induce properties and characteristics1a2a that are beginning to be explored in areas as diverse as anion binding,[3] catalysis,[4] materials,[5] health care,[6] and the kinetic stabilization of supramolecular structures.[7]. The metalated knot, link, and the parent open cyclic helicates display good to very strong halide binding affinities in their central cavities (Ka ≈ 105−1010 M−1 in MeCN).3a The anion-binding properties of the interiors of these metal-coordinated molecular structures led us to investigate their potential as transmembrane ion channels or transporters.[10]. Metal ion-ligand coordination can be used to assemble ionophoric supramolecular structures, and a number of metallo-organic ion transporters have been described.[15] We reasoned that the high anion affinities3a and the rigid shape with internal cavity imposed by the topology of the metalated pentafoil knot 24a and Star of David link 39c could make them suitable candidates for ion transportation (Scheme 1).
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