Surface Charge and the Conductance of Phospholipid Membranes

Abstract

Abstract. Bilayer membranes, formed from various phospholipids, were studied to assess the influence of the charge of the polar head groups on the membrane conductance mediated by neutral "carriers" of cations and anions. The surface charge of an amphoteric lipid, phosphatidyl ethanolamine, was altered by varying the pH, and the surface charge of several lipids was screened by increasing the ionic strength of the solution with impermeant monovalent and divalent electrolytes. The-surface charge should be a key parameter in defining the membrane conductance for a variety of permeation mechanisms; conductance measurements in the presence of carriers may be used to estimate the potential difference, due to surface charge, between the interior of the bilayer and the bulk aqueous phase. The large changes in conductance observed upon varying the surface charge density and the ionic strength agree with those predicted by the Gouy-Chapman theory for an aqueous diffuse double layer. Explicit expressions for the dependence of the membrane conductance on the concentrations of the carrier, the permeant ion, the surface charge density, and the ionic strength are presented.With the development of the artificial phospholipid bilayer membrane,1 2 the relationship between phospholipid composition and permeability properties of the membrane has become accessible to experimental study. The significance of the fluidity of the membrane for solute permeation has been shown.3 Moreover, surface potential measurements on phospholipid monolayers and studies of the electrophoretic mobility of phospholipid dispersions have demonstrated that the presence of a charged polar head group produces a substantial potential at the lipid-solution interface. Such a potential should influence the concentration of ions at the interface, and hence, the permeability properties of the membrane. Studies on phospholipid vesicles4 5 and bilayers6 7 have indeed shown that the anion to cation permselectivity is influenced by the charge of the membrane. Furthermore, the conductance of a bilayer due to the iodide ion or certain weak acids is affected by the charge on the polar head group of the phospholipid.8 9 The present study examines theoretically and experimentally how certain molecules which enhance the membrane conductance, probably by acting as carriers10-12 of particular cations and anions, may be used as "probes" to distinguish between effects of charge and of other variables, such as the fluidity of the bilayer interior and the lipid solubility of the complex.

Surface potential reflected in both gating and permeation mechanisms of sodium and calcium channels of the tunicate egg cell membrane

“…The charge density of le-j80 A2 in the tunicate egg is in good agreement with le-/77 A2 from changes in 9Na and g9 of the giant axon ofa worm Myxicola (Schauf, 1975) or with le-j70 A2 from changes ingNa in the frog node (Vogel, 1974). However, these densities obtained from the excitable membranes in situ are less than those obtained in the charged lipid bilayer membranes of le-f38-60 A2 (McLaughlin et al 1970(McLaughlin et al , 1971Muller & Finkelstein, 1972). In the artificial lipid bilayers, surface charges are mainly derived from the phospholipids.…”

Possible screening of surface charges on crayfish axons by polyvalent metal ions

“…Another possible answer is that axons which do not show evidence of screening simply have a higher percentage of neutral (but amphoteric) lipids in the region of the sodium 'gates'. Regardless of which interpretation is more accurate, an increase in pH ought to bring about both the ionization of a larger percentage of the phospholipid anionic groups and the neutralization of the positive amine groups in some of the phospholipid polar complexes (see McLaughlin et al 1970) and thereby make screening more likely. This expectation is largely fulfilled by findings from studies using frog myelinated nerve.…”

Inactivation of the alamethicin-induced conductance caused by quaternary ammonium ions and local anesthetics.

“…The first method involves the use of the ion carrier nonactin as a probe of membrane surface potential (McLaughlin et al, 1970) in symmetric GMO/PS membranes, and the second involves measurements of the voltage-dependent part of the capacitance in membranes in which one monolayer consists of GMO/PE and the other of GMO/PS (Alvarez and Latorre, 1978). In the first case the surface charge density, cro, was found by measuring the nonactin-induced zero voltage conductance G(0) as a function of ionic strength.…”

“…In the first case the surface charge density, cro, was found by measuring the nonactin-induced zero voltage conductance G(0) as a function of ionic strength. The ionic strength was varied by adding LiC1 to both sides of the membrane, keeping the KC1 concentration constant, o" 0 can be obtained by fitting the experimental points with the equation (McLaughlin et al, 1970):…”