Polar group orientation and the electrical properties of lecithin bimolecular leaflets

Abstract

The first part of this paper is concerned primarily with the question of whether the phosphate and trimethylammonium ions in the head groups of a lecithin bimolecular leaflet are oriented in lines normal to, or in a plane parallel to the leaflet. This problem has been approached by comparing the measured electrokinetic potentials with those to be expected theoretically from these two orientations. In order to calculate the potentials the equations have been derived for parallel sheets of fixed positive and negative charges freely perfused by an aqueous solution of univalent ions. It has been concluded, from comparison of the theoretical and measured electrokinetic potentials that the phosphate and trimethylammonium groups are in a plane parallel to the leaflet. In the second part of the paper the frequency dependence of the capacitance of the lecithin bimolecular leaflet in aqueous solution has been considered in terms of a three-phase dielectric, i.e. layers of hydrocarbon, phosphatidyl chorine and aqueous solution. Previously obtained experimental results have been extended to show that between approximately 5 × 10⁻³ and 10⁷ c/s the capacitance of the leaflet is constant. Taking into consideration the polar group orientation deduced from the first part of this paper, probable values for the capacitance and conductance of the phosphatidyl choline layer were estimated. Substitution of these values into the electrical equations for the three phase dielectric gives capacitances and conductances for the whole system which are consistent with the observed results. It is concluded that the capacitance and conductance (measured normal to their surfaces) of the lecithin leaflets are controlled almost entirely by the hydrocarbon region and that the polar groups make a negligible contribution.