THE ADSORPTION OF PHLORETIN TO LIPID MONOLAYERS AND BILAYERS CANNOT BE EXPLAINED BY LANGMUIR ADSORPTION-ISOTHERMS ALONE

Phloretin and its analogs adsorb to the surfaces of lipid monolayers a nd bilayers and decrease the dipole potential. This reduces the conduc tance for anions and increases that for cations on artificial and biol ogical membranes. The relationship between the change in the dipole po tential and the aqueous concentration of phloretin has been explained previously by a Langmuir adsorption isotherm and a weak and therefore negligible contribution of the dipole-dipole interactions in the lipid surface. We demonstrate here that the Langmuir adsorption isotherm al one is not able to properly describe the effects of dipole molecule bi nding to lipid surfaces-we found significant deviations between experi mental data and the fit with the Langmuir adsorption isotherm. We pres ent here an alternative theoretical treatment that takes into account the strong interaction between membrane (monolayer) dipole field and t he dipole moment of the adsorbed molecule. This treatment provides a m uch better fit of the experimental results derived from the measuremen ts of surface potentials of lipid monolayers in the presence of phlore tin. Similarly, the theory provides a much better fit of the phloretin -induced changes in the dipole potential of lipid bilayers, as assesse d by the transport kinetics of the lipophilic ion dipicrylamine.