Carrier-mediated transport in model membranes monitored by Eu(III) fluorescence

The kinetics of Eu(III) ion transport between hydrophobic and hydrophilic phases using the ion carrier lasalocid is studied in detail using environment-sensitive Eu(III) ion excitation spectroscopy. The technique utilizes the environment-sensitive shift of the narrow 7F0→5D0 0–0 transition absorption line and the resolution of a tunable dye laser to monitor the kinetics of the ion in each distinct environment. Forward and reverse rate constants for the ion partition between an aqueous phase and a hydrophobic chloroform phase are similar to those for an ion partition between an aqueous phase and the more ordered bilayer membrane phase. Bimolecular rate constants are determined using limiting ion and ionophore conditions.