Ripple-like instability in the simulated gel phase of finite size phosphocholine bilayers

TL;DR

This study uses molecular dynamics simulations to explore the ripple phase in phosphatidylcholine bilayers, revealing size-dependent phase behavior and a new instability mechanism in large systems.

Contribution

It demonstrates that the ripple phase is not stable in large bilayer systems under common conditions and proposes a mechanism for the observed topographic instability.

Findings

Large systems exhibit a disordered gel phase with ripple-like features.

The ripple phase stability depends on system size and thermal history.

A mechanism for ripple-like instability in large bilayers is proposed.

Abstract

Atomistic molecular dynamics simulations have reached a degree of maturity that makes it possible to investigate the lipid polymorphism of model bilayers over a wide range of temperatures. However if both the fluid $L_{\alpha}$ and tilted gel $L_{\beta'}$ states are routinely obtained, the $P_{\beta'}$ ripple phase of phosphatidylcholine lipid bilayers is still unsatifactorily described. Performing simulations of lipid bilayers made of different numbers of DPPC (1,2-dipalmitoylphosphatidylcholine) molecules ranging from 32 to 512, we demonstrate that the tilted gel phase $L_{\beta'}$ expected below the pre-transition cannot be obtained for large systems ($>$ 94 DPPC molecules) through common simulations settings or temperature treatments. Large systems are instead found in a disordered gel phase which display configurations, topography and energies reminiscent from the ripple phase $P_{\beta'}$ observed between the pretransition and the main melting transition. We show how the state of the bilayers below the pretransition can be controlled and depends on thermal history and conditions of preparations. A mechanism for the observed topographic instability is suggested.