P-314 - Electron-transfer reactions within lipid bilayers take place in a medium whose averaged dielectric constant is higher than commonly assumed

Abstract

Biochemical reactions occurring within lipid bilayers take place in a heterogeneous medium. A fluid lipid bilayer is a highly stratified structure with a distinct trans-bilayer molecular profile, e.g. the fluidity and polarity gradients, which in turn may influence the roles of biomembranes as selective barriers or functional arrays. Still, the analysis of electron transfer and redox reactions taking place in membranes is based in a theory that assumes the solvent as a continuum and whose dielectric constant is analogous to that of apolar methylenic solvents. We present the measurement of equivalent dielectric constants and their thermal behavior, for POPC and mixed POPC/cholesterol bilayers, using the pyrene (Py) Ham Effect. To do so, the emission spectra must exclude experimental artifacts from Py fluorescence in liposomes. Given that Py locates in the ordered section of bilayers, it probes dielectric constants averaged transversally in space (the long axis of Py, 9.2 A) and laterally in time (due to Py lateral diffusion during its lifetime 150 ns). At 20 °C, POPC bilayers display a dielectric constant analogous to 1-propanol which is much higher than that of an apolar solvent. Also, POPC bilayers exhibit higher dielectric constant than the mixtures at high cholesterol content.