Theoretical study of electronic transition energies of a molecule adsorbed at the lipid membrane–water interface

Abstract

A simple theoretical model describing the variation of the electronic transition energies for a molecule adsorbed at the lipid membrane–water interface has been developed. The model divides this region into three zones: the hydrophobic core, a region containing the polar head groups and an external aqueous phase. The model also takes into account the possible presence of freely moving ions in the aqueous phase as well as a spontaneous partial polarization of the polar region. The guest molecule can be located in any one of these zones; detailed calculations and explicit analytical formulae are given for the case when the guest molecule is in the polar region. Preliminary experiments have been performed using two different fluorescent dyes adsorbed in a lipid matrix of small phospholipid vescicles. The measurements were made by suspending the vescicles in water–acetonitrile mixtures in order to study the fluorescence intensity and frequency shifts as a function of the external dielectric constant. A comparison between calculated and measured frequency shifts suggests a possible contribution of the spontaneous polarization of the membrane surface together with the usual polarization energy as a rationale of the experimental findings.