Interaction of phloretin with lipid monolayers: Relationship between structural changes and dipole potential change

Phloretin is known to adsorb to lipid surfaces and alters the dipole potent ial of lipid monolayers and bilayers. Its adsorption to biological and arti ficial membranes results in a change of the membrane permeability for a var iety of charged and neutral compounds. In this respect phloretin represents a model substance to study the effect of dipole potentials on membrane per meability. In this investigation we studied the interaction of phloretin wi th monolayers formed of different lipids in the liquid-expanded and the con densed state. Phloretin integrated into the monolayers as a function of the aqueous concentration of its neutral form, indicated by an increase of the surface pressure in the presence of phloretin. Simultaneous recording of t he surface potential of the monolayers allowed us to correlate the degree o f phloretin integration and the phloretin-induced dipole potential change. Increasing the surface pressure decreased the phloretin-induced shift of th e isotherms, but did not influence the phloretin-induced surface potential change. This means that phloretin adsorption to the lipid surface can occur without affecting the lipid packing. The surface potential effect of phlor etin is accompanied by a change of the lipid dipole moment vector dependent on the lipid packing. This means that the relation between the surface pot ential change and the lipid packing cannot be described by a static model a lone. Taking into account the deviations of the surface potential change ve rsus molecular area isotherms of the experimental data to the theoretically predicted course, we propose a model that relates the area change to the d ipole moment in a dynamic manner. By using this model the experimental data can be described much better than with a static model.