Phase diagrams and ordering in charged membranes: Binary mixtures of charged and neutral lipids

TL;DR

This paper models the phase behavior of charged and neutral lipid mixtures in membranes, revealing how charge and salt influence phase separation and the main-transition temperature, with implications for biological membrane organization.

Contribution

It introduces a theoretical model that incorporates the effects of charge, lipid saturation, and salt on membrane phase behavior, extending previous neutral lipid studies.

Findings

Charged lipids lower the main-transition temperature.

Charge suppresses membrane phase separation.

Salt modifies phase behavior and lipid area temperature dependence.

Abstract

We propose a model describing the phase behavior of two-component membranes consisting of binary mixtures of electrically charged and neutral lipids. We take into account the structural phase transition (main-transition) of the hydrocarbon chains, and investigate the interplay between this phase transition and the lateral phase separation. The presence of charged lipids significantly affects the phase behavior of the multi-component membrane. Due to the conservation of lipid molecular volume, the main-transition temperature of charged lipids is lower than that of neutral ones. Furthermore, as compared with binary mixtures of neutral lipids, the membrane phase separation in binary mixtures of charged lipids is suppressed, in accord with recent experiments. We distinguish between two types of charged membranes: mixtures of charged saturated lipid/neutral unsaturated lipid and a second case of mixtures of neutral saturated lipid/charged unsaturated lipid. The corresponding phase behavior is calculated and shown to be very different. Finally, we discuss the effect of added salt on the phase separation and the temperature dependence of the lipid molecular area.