Abstract
In this study we derived a model for a multicomponent lipid monolayer in contact with an aqueous solution by means of a generalized classical density functional theory and Monte Carlo simulations. Some of the important biological lipid systems were studied as monolayers composed of head groups with different shapes and charge distributions. Starting from the free energy of the system, which includes the electrostatic interactions, additional internal degrees of freedom are included as positional and orientational entropic contributions to the free energy functional. The calculus of variation was used to derive Euler–Lagrange equations, which were solved numerically by the finite element method. The theory and Monte Carlo simulations predict that there are mainly two distinct regions of the electric double layer: (1) the interfacial region, with thickness less than or equal to the length of the fully stretched conformation of the lipid head group, and (2) the outside region, which follows the usual screening of the interface. In the interfacial region, the electric double layer is strongly perturbed, and electrostatic profiles and ion distributions have functionality distinct to classical mean-field theories. Based purely on Coulomb interactions, the theory suggests that the dominant effect on the lipid head group conformation is from the charge density of the interface and the structured lipid mole fraction in the monolayer, rather than the salt concentration in the system.
Highlights
In biological systems, electrostatic interactions between charged lipid membranes and aqueous electrolyte solutions are of fundamental interest.[1]
The compositional heterogeneous structure known as phase separation emerged spontaneously at lower temperature.[4−6] The formation mechanism and stability of phase separation in lipid membranes have been discussed in relation to the raft structure, which is believed to be formed in plasma membranes and to play important roles in signal transduction and membrane trafficking.[7,8]
The primary focus of the present work is to investigate the structure of the electric double layer (EDL) around the phospholipid monolayer
Summary
Electrostatic interactions between charged lipid membranes and aqueous electrolyte solutions are of fundamental interest.[1]. Apart from cDFT, Monte Carlo (MC) simulations were often used to test theoretical predictions.[23,52] By MC simulations, various static properties can be deduced, including the thermodynamics and structure of model systems.[49] While molecular dynamics (MD) simulations can be used to include further effects into the description of the interface wetted with water molecules and the soft-matter aspect of the interface,[50,53,54] the computational demand is very high.[36] both MC and MD simulations are limited to a small range of system compositions, e.g., high salt concentrations, small patches of predetermined mole fractions of lipids in the bilayers, etc This generates a demand for mean-field and general cDFT theories which can be utilized to recover thermodynamics of lipid systems for various system compositions and applications.[43].
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