Studies on Electrokinetic Effects Across Supported Mixed Lipid Membranes

We considered artificial mixed lipid membranes, supported by a pyrex sintered G4 glass disc and measured the hydraulic permeability, electroosmosis, the streaming potential and current of acetone in this system. The phenomenological coefficients have been calculated by applying the formalism of non-equilibrium thermodynamics. The cross coefficients L12 and L2i were found nearly equal showing the validity of the Onsager reciprocal relation for the system studied. The thermodynamic fluxes and forces were found to be linearly related. The maximum of the electrokinetic energy conversion of electroosmosis and of the streaming potential has been calculated. Introduction In previous studies, single lipid membranes, supported by pyrex sintered G4 glass discs, have been considered and electrokinetic effects of water, acetone and their mixtures of various compositions have been measured [1-3], Water and acetone are present in most bookes of animals and the plasma membrane, surrounding a biological cell contains a mixture of lipids, proteins, enzymes etc. In order to have some basic information of the transfer of liquids across cellular membranes, artificial membranes, having two lipids together, were prepared. The lipid pairs chosen were cephalin + lecithin, lecithin + phosphatidyl inositol (inositol), inositol + phosphatidyl serine (serine), serine + lecithin, cephalin + inositol and cephalin 4serine. The study is« important from a physiological and biochemical point of view. J. Non-Equilib. Thermodyn. , Vol. 12, 1987, No. 3 Copyright © 1987 Walter de Gruyter Berlin · New York j98 S.A. Rizvi, S.B. Zaidi 1. Experimental (a) Purification of Chemicals: é Phospholipids (Bovine Brain Koch Ltd., England) acetone, methanol and Chloroform (A. R., B. D. H.) were purified in the same way described earlier [1,2]. The pure lipids were stored in a vacuum desiccator in order to prevent decomposition. The densities of the (pure) acetone, chloroform and methanol used were d = 0.7680 (0.7682), d = 1.4707 (1.4704) and d = 0.7824 (0.7825) respectively. The values given in brackets are those from literature [4]. (b) Membrane preparation and measurement of electrokinetic effects: An electroosmotic cell with a pyrex sintered G4 glass disc fused in the middle [1] was washed several times successively with hot 1:1 HNO3 and water. It was then dried for 24 hours for the deposition of the lipid layer. The lipid solution, used for incorporation, was prepared by dissolving equal weights of two lipids (5 mgs of each). These were dissolved in 10 ml of a chloroform + methanol mixture to which 2 mgs, i.e. 20% of the lipids' weight of tetradecane was added so as to prevent solidification of the lipids [5]. The total weight of the two lipids always has been equal to that used for the incorporation of a single lipid [1]. The procedure adopted to deposit the lipids has been the same as described in [1,6]. The homogeneity of the lipid layer was ensured by telescopic examination. The amount of lipid mixture deposited per unit area of the disc was 0.238 mg. Electrokinetic effects, i. e. the hydraulic permeability, electroosmosis, the streaming potential, current and resistance of the permeant between the electrodes were measured in the usual way [1,2]. All necessary precautions were taken and possible sources of error avoided [7]. The electroosmotic cell was accommodated in a thermostat maintained at 40 ± 0.1 °C. The results were reproducible to ±0.5%. Data are given in Tab. 1 below. Tab. 1: L-coefficients of acetone across mixed lipid membranes.