Fluctuations in lipid bilayers: Are they understood?

TL;DR

This review discusses recent simulation studies of lipid bilayer fluctuations, highlighting successes of elastic models in fluid phases and challenges in gel phases and under tension, with insights into tension-fluctuation relationships.

Contribution

It provides a comprehensive analysis of membrane fluctuation models, comparing theoretical predictions with simulation results across different membrane phases and tension conditions.

Findings

Elastic models accurately describe tensionless fluid membranes

Gel phase membranes exhibit ripple structures affecting fluctuations

Fluctuation modes are softer under tension than predicted by theory

Abstract

We review recent computer simulation studies of undulating lipid bilayers. Theoretical interpretations of such fluctuating membranes are most commonly based on generalized Helfrich-type elastic models, with additional contributions of local "protrusions" and/or density fluctuations. Such models provide an excellent basis for describing the fluctuations of tensionless bilayers in the fluid phase at a quantitative level. However, this description is found to fail for membranes in the gel phase and for membranes subject to high tensions. The fluctuations of tilted gel membranes show a signature of the modulated ripple structure, which is a nearby phase observed in the pretransition regime between the fluid and tilted gel state. This complicates a quantitative analysis on mesoscopic length scales. In the case of fluid membranes under tension, the large-wavelength fluctuation modes are found to be significantly softer than predicted by theory. In the latter context, we also address the general problem of the relation between frame tension and the fluctuation tension, which has been discussed somewhat controversially in recent years. Simulations of very simple model membranes with fixed area show that the fluctuations should be controlled by the frame tension, and not by the internal tension.