PERMEATION OF PHLORETIN ACROSS BILAYER-LIPID MEMBRANES MONITORED BY DIPOLE POTENTIAL AND MICROELECTRODE MEASUREMENTS

The transmembrane diffusion of phloretin across planar bilayer lipid m embranes is studied under steady-state conditions. Diffusion restricti ons and adsorption related effects are measured independently. The ads orption of aligned phloretin dipoles generates a change in the intrins ic dipole potential difference between the inner and outer leaflets of the lipid bilayer. It is monitored by capacitive current measurements carried out with a direct current (dc) bias. The variation of the int ramembrane electric field indicates a saturation of the binding sites at the membrane interface. In contrast, pH profile measurements undert aken in the immediate membrane vicinity show a constant membrane perme ability. If phloretin binding and transmembrane diffusion are treated as two competitive events rather than subsequent steps in the transpor t queue the contradictory results become explainable. A mathematical m odel is developed where it is assumed that diffusing phloretin molecul es are randomly oriented, i.e., that they do not contribute to the int rinsic membrane potential. Only the dipoles adsorbing onto the membran e are oriented. Based on these theory the membrane permeability is cal culated from the capacitive current data. It is found to agree very we ll with the permeability deduced from the microelectrode measurements.