Repositioning Chloride Transmembrane Transporters: Transport of Organic Ion Pairs

Given the biological importance of organic cations, the facilitated transport of organic ion pairs could find many applications. Calix[6]arene tris(thio)ureas, which possess a cavity that can accommodate primary ammonium ions, can not only act as carriers for Cl−/NO3− antiport but can also perform the cotransport of PrNH3Cl. Transport was monitored by fluorescence spectroscopy and the presence of the different species inside the vesicles was characterized by 1H and 35Cl NMR experiments involving shift reagents. The cotransport of PrNH3Cl was also observed by receptors deprived of a cavity, but the presence of the cavity conveys an advantage, as the cotransport by calix[6]arenes was observed to be more efficient than the Cl−/NO3− antiport, which is not the case with receptors without a cavity. The role played by the cavity was further highlighted by the disappearance of this advantage when using a bulky ammonium ion, which cannot be complexed within the cavity.