Article

Internal electrostatic potentials in bilayers: measuring and controlling dipole potentials in lipid vesicles. Biophys. J. 65: 289-299

Department of Chemistry, University of Virginia, Charlottesville 22901.
Biophysical Journal (Impact Factor: 3.97). 08/1993; 65(1):289-99. DOI: 10.1016/S0006-3495(93)81051-8
Source: PubMed

ABSTRACT

The binding and translocation rates of hydrophobic cation and anion spin labels were measured in unilamellar vesicle systems formed from phosphatidylcholine. As a result of the membrane dipole potential, the binding and translocation rates for oppositely charged hydrophobic ions are dramatically different. These differences were analyzed using a simple electrostatic model and are consistent with the presence of a dipole potential of approximately 280 mV in phosphatidylcholine. Phloretin, a molecule that reduces the magnitude of the dipole potential, increases the translocation rate of hydrophobic cations, while decreasing the rate for anions. In addition, phloretin decreases the free energy of binding of the cation, while increasing the free energy of binding for the anion. The incorporation of 6-ketocholestanol also produces differential changes in the binding and translocation rates of hydrophobic ions, but in an opposite direction to those produced by phloretin. This is consistent with the view that 6-ketocholestanol increases the magnitude of the membrane dipole potential. A quantitative analysis of the binding and translocation rate changes produced by ketocholestanol and phloretin is well accounted for by a point dipole model that includes a dipole layer due to phloretin or 6-ketocholestanol in the membrane-solution interface. This approach allows dipole potentials to be estimated in membrane vesicle systems and permits predictable, quantitative changes in the magnitude of the internal electrostatic field in membranes. Using phloretin and 6-ketocholestanol, the dipole potential can be altered by over 200 mV in phosphatidylcholine vesicles.

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    • "Temptatively, the inner interface is the plane at which dipoles of the carbonyl groups and water polarized by them are aligned to define the dipole potential (Smaby and Brockman 1990; Franklin and Cafiso 1993; Brockman 1994; Diaz et al. 1999). The outer interface is the ideal plane at which the zeta potential is measured to determine the surface charge potential (Seelig et al. 1987; McLaughlin 1989; Lairion and Disalvo 2009). "
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    ABSTRACT: In order to give a physical meaning to each region of the membrane we define the interphase as the region in a lipid membrane corresponding to the polar head groups imbibed in water with net different properties than the hydrocarbon region and the water phase. The interphase region is analyzed under the scope of thermodynamics of surface and solutions based on the definition of Defay-Prigogine of an interphase and the derivation that it has in the understanding of membrane processeses in the context of biological response. In the view of this approach, the complete monolayer is considered as the lipid layer one molecule thick plus the bidimensional solution of the polar head groups inherent to it (the interphase region). Surface water activity appears as a common factor for the interaction of several aqueous soluble and surface active proteins with lipid membranes of different composition. Protein perturbation can be measured by changes in the surface pressure of lipid monolayers at different initial water surface activities. As predicted by solution chemistry, the increase of surface pressure is independent of the particle nature that dissolves. Therefore, membranes give a similar response in terms of the determined surface states given by water activity independent of the protein or peptide.
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    • "The dipole potential of THP-1 cells was modulated by the addition of 6-ketocholestanol (6-KC, see Fig. 1) [9]. As a reference probe, we used di-8-ANEPPS [6] (Fig. 1). "

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    • "Interaction of Polylysines with the Surface of Lipid Membranes component cannot be measured directly but its deviations may be controlled indirectly by different dipole sensitive probes [55] [57] [59] [60] or by subtracting electric field changes in the diffuse part of EDL (zeta-potentials) from total BP that comes, for instance, from direct Volta potential measurements at lipid monolayer [37] [55] or from IFC measurements with planar BLM. The latter method, described in detail in Ref. [32], assumes that the planar BLM conductivity remains negligible in the presence of adsorbed polymer and this condition has to be carefully controlled because PL of high-molecular weight decrease the BLM stability to applied external voltage [14]. "
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    ABSTRACT: The topic correlates electrostatic effects induced by polylysine (PL) adsorption at the lipid membrane surface with data of alternative methods sensitive to lipid bilayer structure. Comparison of electrokinetic data for liposomes from anionic lipids (cardiolipin, phosphatidylserine) and results of boundary potential (BP) measurements with lipid membranes shows effects in two opposite directions: fast positive changes of BP due to adsorption of polycations at the outer membrane surface and slow negative changes that can be attributed to alteration of the dipole component of BP. The latter effect does not depend on the polymer length and may be caused by lipid interaction with lysine as a basic unit of these polypeptides. Molecular dynamic simulation points out the possible mechanism of the dipole effect, which could be caused by reduced number of H-bonds to PO4 groups upon the lysine adsorption. Atomic force microscopy visualized the geometry of clusters formed by PL of different lengths at the lipid bilayer. Isotherm titration calorimetry and the technique of lipid monolayers reveal the similarity in polypeptide and inorganic multivalent cation effects on the lateral lipid condensation accompanied by dipole effects.
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