Lateral phase separation in mixtures of lipids and cholesterol systems

TL;DR

This paper develops phenomenological models to understand phase separation and raft formation in biological membranes by analyzing lipid and cholesterol mixtures, producing phase diagrams consistent with experimental observations.

Contribution

It introduces new models linking membrane composition and structure, explaining phase transitions and raft formation in lipid-cholesterol systems.

Findings

Phase diagrams match experimental data semi-quantitatively.

Cholesterol influences membrane phase transitions as an external field.

Models predict behavior in ternary lipid systems.

Abstract

In an effort to understand ``rafts'' in biological membranes, we propose phenomenological models for saturated and unsaturated lipid mixtures, and lipid-cholesterol mixtures. We consider simple couplings between the local composition and internal membrane structure, and their influence on transitions between liquid and ``gel'' membrane phases. Assuming that the gel transition temperature of the saturated lipid is shifted by the presence of the unsaturated lipid, and that cholesterol acts as an external field on the chain melting transition, a variety of phase diagrams are obtained. The phase diagrams for binary mixtures of saturated/unsaturated lipids and lipid/cholesterol are in semi-quantitative agreement with the experiments. Our results also apply to regions in the ternary phase diagram of lipid/lipid/cholesterol systems.