Fluctuations and Elastic Properties of Lipid Membranes in the Tilted Gel State: A Coarse-Grained Monte Carlo Study

TL;DR

This study uses Monte Carlo simulations to analyze the elastic and fluctuation properties of lipid membranes in the tilted gel phase, revealing anisotropic behavior and changes in spontaneous curvature compared to fluid membranes.

Contribution

It provides the first detailed simulation-based analysis of lipid membrane fluctuations and elastic properties specifically in the tilted gel phase, connecting to theoretical models.

Findings

Spontaneous curvature changes sign from fluid to gel phase.

Height fluctuations are suppressed and highly anisotropic in the gel phase.

Ripple-like soft modes are observed along the tilt direction.

Abstract

We study the stress distribution profiles and the height and thickness fluctuations of lipid membranes in the tilted gel state by Monte Carlo simulations of a generic coarse-grained model for lipid membranes, which reproduces many known properties of DPPC bilayers. The results are related to the corresponding properties of fluid membranes, and to theoretical predictions for crystalline and hexatic membranes. One striking observation is that the spontaneous curvature of the monolayers changes sign from the fluid to the gel phase. In the gel phase, the long-wavelength height fluctuations are suppressed, and the fluctuation spectrum is highly anisotropic. In the direction of tilt, it carries the signature of soft modes that are compatible with the wavelength of the ripple phase, which emerges in the transition region between the fluid and the gel state. In the direction perpendicular to the tilt, the thickness fluctuations are almost entirely suppressed, and the height fluctuations seem to be dominated by an interfacial energy, i.e., by out-of-layer fluctuations, up to length scales of tens of nanometers.