Angew. Chem. Int. Ed. (2011)

Credit: © 2011 WILEY

Anion–π interactions and halogen bonds are weaker and much less common than their cation–π interactions and hydrogen bond counterparts. Yet when it comes to carrying ions through lipid bilayer membranes, stronger binding does not necessary lead to better transport. Using a series of calixarenes as ion transporters, Stefan Matile and co-workers at the University of Geneva and the Politecnico di Milano have now unravelled the contributions of anion–π interactions, halogen bonds and hydrogen bonds to anion transport.

The researchers prepared calixarenes that bind anions through one rim and counter-cations through the other. Transport only occurred when the cation was tetramethyl ammonium — behaviour consistent with counter-ion activations previously observed with other synthetic transporters. The anion-binding rim consists of arene rings, which, through substitutions between fluorine, iodine and hydrogen atoms at specific positions, were devised to form anion–π interactions, halogen bonding of various strengths, or hydrogen bonding.

The calixarenes' transport abilities were determined using vesicles made of lipid bilayer membranes that encapsulate a pH-sensitive fluorophore in a basic solution. Calixarenes transported chloride and hydroxide anions through the membrane in opposite ways, causing a pH variation that was observed by fluorescence. The best transport system was the one featuring anion–π interactions. Halogen binding of the anions was initially too strong for transport to occur, but this was addressed by weakening the interaction or increasing its distance — which shows that anions can be carried through membranes using halogen, rather than hydrogen, bonds.