Abstract
The lipid bilayer is a basic building block of biological membranes and can be pictured as a barrier separating two compartments filled with electrolyte solution. Artificial planar lipid bilayers are therefore commonly used as model systems to study the physical and electrical properties of the cell membranes in contact with electrolyte solution. Among them the glycerol-based polar phospholipids which have dipolar, but electrically neutral head groups, are most frequently used in formation of artificial lipid bilayers. In this work the electrical properties of the lipid layer composed of zwitterionic lipids with non-zero dipole moments are studied theoretically. In the model, the zwitterionic lipid bilayer is assumed to be in contact with aqueous solution of monovalent salt ions. The orientational ordering of water, resulting in spatial variation of permittivity, is explicitly taken into account. It is shown that due to saturation effect in orientational ordering of water dipoles the relative permittivity in the zwitterionic headgroup region is decreased, while the corresponding electric potential becomes strongly negative. Some of the predictions of the presented mean-field theoretical consideration are critically evaluated using the results of molecular dynamics (MD) simulation.
Highlights
The lipid bilayer is a basic building block of cell membranes
In Modified Langevin-Poisson-Boltzmann (MLPB) model the function P(x) is steeply increasing in the vicinity of x = 0 plane (Figure 6, plot A) which is a consequence of the exponential Boltzmann factor in the function P(x) (Equation 13)
The discrepancy between the predictions of MLPB model and molecular dynamic simulation (MD) simulations arises due to the fact that the finite volumes of lipid headgroups and finite volumes of ions and water molecules are not considered in MLPB model
Summary
The lipid bilayer is a basic building block of cell membranes. the cell membrane is a highly heterogeneous structure, composed of lipids, proteins, carbohydrates and other components [1,2,3], a large number of physical properties of biological (cell) membrane which are important in physiological processes [4,5] can be studied in pure planar lipid bilayer systems [6,7]. Electrolyte solution is basically an aqueous solution of salt composed of water molecules (Figure 1) and monovalent positively and negatively charged ions Such experimental setup is ordinarily used to determine various mechanical and electrical properties of planar lipid bilayers [9,11,12]. In this paper the effect of the nonhomogeneous volume charge distribution in the headgroup region of the planar dipolar lipid layer on the space dependent electric potential and permittivity is presented. The parameters p0 and n0w /NA are 3.1 Debye and 55 mol/l, respectively
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