Unusual behavior of ion transport mediated by polyene antibiotics. Activation energies for the exchange of Na+ ions through liposomal membranes studied by 23Na-NMR spectroscopy.

23Na-NMR spectroscopy has been applied to study the transport of Na+ ions across unilamellar vesicle membranes mediated by ionophores. The ionophores used were amphotericin B and nystatin. Also, monensin, lasalocid A and gramicidin D were included for the sake of comparison since the transport processes of these ionophores are well characterized as carrier-, carrier- and channel-types, respectively. The 23Na-NMR techniques employed were 2D-EXSY (exchange spectroscopy) and the ID time-course monitoring techniques, and the measurement exchange rates. These techniques were also applied at different temperatures, and the activation parameters were determined for the ionophore-mediated Na+ exchange. These activation parameters disclosed an unusual behavior of amphotericin B and nystatin. That is, cation transport through the membrane is decreased on increasing the temperature of measurement, resulting in negative values for the apparent enthalpy of activation. Such unusual behavior is related to the fluidity of the model membrane and to intermolecular interactions in the membrane.