The molecular organisation of bimolecular lipid membranes. The dielectric structure of the hydrophilic/hydrophobic interface

Abstract

Improvements to a previously described very low-frequency impedance-measuring technique have now allowed the characterisation of a third, electrically distinct, type of substructural region in phosphatidylcholine biomolecular lipid membranes. This region was found to have properties intermediate to those of the hydrophobic (hydrocarbon) layer and the regions containing the polar heads of the phosphatidylcholine molecules. Its properties are consistent with it being associated with the oxygen-rich carboxyl ester portions of the phosphatidylcholine molecules which lie at the hydrophilic/hydrophobic interface. We will refer to these regions in the membrane as the acetyl regions. The individual properties of the three distinct types of region in the phosphatidylcholine membranes were determined at KCl electrolyte concentrations of 1, 10, 100 and 1000 mM. It was found that with increasing KCl concentration: (a) The capacitance, CH, of the hydrophobic region increased slightly, indicating a decrease in the thickness of this region. (b) The conductance, GH, of this hydrophobic region increased by a factor of 20 in going from 1 to 1000 mM electrolyte. (c) The capacitance of the acetyl region was independent of KCl concentration although its conductance increased 5-fold over the range 1-1000 mM KCl. (d) The volume-specific electrical properties of the region containing the polar heads appeared to be essentially independent of KCl concentration. However, a change in thickness of these regions was observed which was consistent with the cholinephosphate dipole being oriented normal to the bilayer surface in 1 mM KCl and parallel to the surface in 1000 mM KCl external solutions.