Effects of passive phospholipid flip-flop and asymmetric external fields on bilayer phase equilibria

TL;DR

This paper develops a theoretical framework to understand how passive phospholipid flip-flop and external symmetry-breaking fields influence bilayer membrane phase behaviour, revealing stable asymmetric states and explaining experimental observations.

Contribution

It introduces a model predicting phase equilibria changes due to passive flip-flop and external fields, highlighting the emergence of stable asymmetric membrane states.

Findings

Passive flip-flop can induce stable asymmetric phase states.

External fields can stabilize strongly asymmetric membrane configurations.

The theory explains experimental observations of flip-flop effects on phase behaviour.

Abstract

Compositional asymmetry between the leaflets of bilayer membranes modifies their phase behaviour, and is thought to influence other important features such as mechanical properties and protein activity. We address here how phase behaviour is affected by passive phospholipid \textit{flip-flop}, such that the compositional asymmetry is not fixed. We predict transitions from "pre flip-flop" behaviour to a restricted set of phase equilibria that can persist in the presence of passive flip-flop. Surprisingly, such states are not necessarily symmetric. We further account for external symmetry-breaking, such as a preferential substrate interaction, and show how this can stabilise strongly asymmetric equilibrium states. Our theory explains several experimental observations of flip-flop mediated changes in phase behaviour, and shows how domain formation and compositional asymmetry can be controlled in concert, by manipulating passive flip-flop rates and applying external fields.