Fluctuations and elastic properties of lipidmembranes in the gel Lβ′ state: a coarse-grained Monte Carlo study

Abstract

We study the stress distribution profiles and the height and thickness fluctuations of lipid membranes in the gel L_β′ state by Monte Carlo simulations of a generic coarse-grained model for lipid membranes, which reproduces many known properties of dipalmitoylphosphatidyncholine (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 with wavelengths compatible to those of the ripple phase P_β′, 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 nanometres.