Phase behavior of two-component lipid membranes: theory and experiments

TL;DR

This paper develops a phenomenological theory for the ripple phase in lipid membranes, accurately modeling its structure and phase diagrams, and validates it with experimental x-ray diffraction data.

Contribution

It introduces a coupled order parameter model that explains the ripple phase features and predicts a new variant, unifying theoretical and experimental phase diagrams.

Findings

Model reproduces structural features of ripple phase

Calculated phase diagrams match x-ray diffraction results

Predicts a novel ripple phase variant

Abstract

The structure of the ripple phase of phospholipid membranes remains poorly understood in spite of a large number of theoretical studies, with many experimentally established structural features of this phase unaccounted for. In this article we present a phenomenological theory of phase transitions in single- and two-component achiral lipid membranes in terms of two coupled order parameters -- a scalar order parameter describing {\it lipid chain melting}, and a vector order parameter describing the {\it tilt of the hydrocarbon chains} below the chain-melting transition. This model reproduces all the salient structural features of the ripple phase, providing a unified description of the phase diagram and microstructure. In addition, it predicts a variant of this phase which does not seem to have been experimentally observed. Using this model we have calculated generic phase diagrams of two-component membranes. We have also determined the phase diagram of a two-component lipid membrane from x-ray diffraction studies on aligned multilayers. This phase diagram is found to be in good agreement with that calculated from the model.