Crown ethers transmembrane transport

Electronic signals in nerve cells travel by means of metal-ion transport laterally in and out of the axon. Such transmembrane transport of ions is fundamental to cell biology, and attempts to mimic it artificially laid the foundation of supramolecular chemistry. Early studies in molecular recognition by Lehn in the 1970s explored the use of crown ethers as mimics of cyclic peptide ionophores like valinomycin, which bind cations selectively in their internal cavities. Natural ionophores act as antibiotics by upsetting the ionic balance across bacterial cell walls. 

Functional artificial ion channels have been reported which illustrate the general criteria. The most obvious course is to prepare oligopeptides with hi helical content (S). Other reported systems are bas on cyclo xtrin (6), polymeric crown ethers (7), and "bouquet" shaped crown ether and cyclodextrin motifs (8). One of the most active systems is a simple tris-crown ether derivative repented by Gokel for the transport of sodium ions(9). All of these systems envisage a uni- or bi-molecular transmembrane structure, similar to the gramicidin structurd paradigm. 

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