Concentration fluctuations and phase transitions in coupled modulated bilayers

TL;DR

This paper models the formation of finite-size lipid domains in bilayers by coupling compositional and order parameters, analyzing fluctuations and phase transitions in coupled modulated bilayers.

Contribution

It introduces a coupled bilayer model with a scalar and vector order parameter, linking microemulsion behavior to lipid domain formation.

Findings

Finite-size domains arise from coupling-induced microemulsion free-energy.

Above transition temperature, concentration fluctuations exhibit specific static and dynamic properties.

Below transition, micro-phase separation leads to distinct structures with characteristic fluctuation behaviors.

Abstract

We consider the formation of finite-size domains in lipid bilayers consisting of saturated and hybrid lipids. First, we describe a monolayer model that includes a coupling between a compositional scalar field and a two-dimensional vectorial order-parameter. Such a coupling yields an effective two-dimensional microemulsion free-energy for the lipid monolayer, and its characteristic length of compositional modulations can be considered as the origin of finite-size domains in biological membranes. Next, we consider a coupled bilayer composed of two modulated monolayers, and discuss the static and dynamic properties of concentration fluctuations above the transition temperature. We also investigate the micro-phase separation below the transition temperature, and compare the micro-phase separated structures with statics and dynamics of concentration fluctuations above the transition.