Theoretical models for quasi-two-dimensional mesomorphic monolayers and membrane bilayers

Abstract

Simple models are developed for the first-order (main) gel–fluid phase transition in lipid mono-and bilayers. The lipid chains are assumed to lie on a triangular lattice in both phases and to interact via anisotropic van der Waals forces. The article begins with a review of previous experimental and theoretical work. Next a two-state Ising-like model is developed and shown to give a good qualitative description of the main phase transition of both mono- and bilayers and of statistical fluctuations about this transition. The article continues with a description of a more quantitative 10-state model for the main bilayer phase transition due to Pink. The polar head interactions are now explicitly taken into account. It is shown that this model is able to account for a variety of experimental results, e.g., Raman scattering data for pure lipid bilayers, phase separations for mixed lipid systems, and ionic permeability data for bilayers. The 10-state model is then extended to include the presence of intrinsic molecules (i.e., cholesterol, anaesthetics, and proteins) dissolved in lipid bilayers. The phase diagrams with their corresponding phase separation regions, the heat of transition, and in some cases the correlations are calculated for such mixed systems using several values of the parameters for the interactions. The penultimate section contains an analysis of the partitioning of anaesthetics between bilayers and the surrounding aqueous medium and its effect on the main phase transition. The article concludes with a discussion of the theoretical results and a summary of recent work.