Anion transport across phospholipid bilayers promoted by a guanidinium calix[4]arene conjugate

The ionophoric activity of a calix[4]arene guanidinium conjugate (4G4Oct) has been investigated in bulk chloroform membrane and mainly in liposomes. In both membrane models 4G4Oct shows high activity in the transport of chloride. The mechanistic evidence suggests a carrier mechanism in which the calixarene binds the anions through electrostatic interactions and hydrogen bonds with the guanidinium moieties. Studies on anion transport in liposomes indicate a peculiar selectivity sequence inverse to the most commonly observed lyotropic series with chloride transported faster than bromide and iodide virtually non-transported. This selectivity sequence together with competitive inhibition of chloride transport by the non-transported ions suggests a strong anion binding to the guanidinium moieties in the membrane environment. The complex formed is lipophilic and is able to shuttle the ion across the membrane. The origin of the observed anion selectivity is suggested to be related to a less favourable ion exchange at the membrane interface of iodide with respect to chloride with OH2, which is the antiported ion in the transport process in liposome. These studies complement the reported ability of 4G4Oct to condensate plasmid DNA and to transfect cells, showing that the calixarene derivative is also able to efficiently transport chloride across a phospholipid bilayer.