Phenomenological Model and Phase Behavior of Saturated and Unsaturated Lipids and Cholesterol

TL;DR

This paper develops a phenomenological model explaining the phase behavior of ternary lipid-cholesterol mixtures, capturing liquid and gel phases and the effects of cholesterol on lipid ordering and phase separation.

Contribution

It introduces a new phenomenological theory that describes both liquid and gel phases in lipid-cholesterol mixtures, elucidating the mechanism of phase separation.

Findings

Cholesterol increases saturated lipid order, promoting phase separation.

Phase diagrams show liquid-liquid separation above the main chain transition.

Below the transition, gel phases coexist with liquid phases.

Abstract

We present a phenomenological theory for the phase behavior of ternary mixtures of cholesterol and saturated and unsaturated lipids, one which describes both liquid and gel phases, and illuminates the mechanism of the behavior. In a binary system of the lipids, the two phase separate when the saturated chains are well ordered, as in the gel phase, simply due to packing effects. In the liquid phase the saturated ones are not sufficiently well ordered for separation to occur. The addition of cholesterol, however, increases the saturated lipid order to the point that phase separation is once again favorable. For the system above the main chain transition of the saturated lipid, we can obtain phase diagrams in which there is liquid-liquid phase separation in the ternary system but not in any of the binary ones, while below that temperature we obtain the more common phase diagram in which a gel phase, rich in saturated lipid, appears in addition to the two liquid phases.