Effect of intermonolayer coupling on the phase behavior of lipid bilayers

A statistical-mechanical lattice model is proposed to describe the acyl-chain main phase transition in a hydrated lipid bilayer. The model is built on a two-dimensional multistate lattice model to describe the intramonolayer interactions within the two separate lipid monolayers of the bilayer. The coupling between the two monolayers is modeled both indirectly by hydrophobic acyl-chain mismatch interactions that ensure compatibility between the two monolayers, and by a direct intermonolayer attractive dispersion force. The nature of the phase transition is studied by computer-simulation methods involving standard Monte Carlo simulation, as well as the extrapolation method of Ferrenberg and Swendsen [Phys. Rev. Lett. 61, 2635 (1988)] and the Lee-Kosterlitz technique [Phys. Rev. Lett. 65, 137 (1990); Phys. Rev. B 43, 3265 (1991)]. It is found that the absence of a phase transition in a set of uncoupled monolayers is restored by a weak intermonolayer interaction. The bilayer properties in the transition region are described with particular emphasis on the lateral density fluctuations and the resulting dynamic bilayer heterogeneity. The theoretical results are discussed in relation to experimental data.