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 H-1 and Cl-35 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.