Chapter six - Monte Carlo Simulations of Lipid Bilayers and Liposomes Using Coarse-Grained Models

Abstract

Abstract Monte Carlo simulations provide some insight into self-assembled aggregates of amphiphiles in aqueous environment. A rather simple solvent-free model, where a molecule is formed by a hydrophilic head segment and some hydrophobic chain segments, is suitable for describing the formation of micelles, stable membranes, and spherical vesicles. Characteristic features of the self-assembled aggregates, such as the elastic properties of bilayers, can be obtained from simulated data. The capability of this simple approach was demonstrated for a surfactant model with three spherical segments. Analyzing vesicle fluctuations by Monte Carlo simulations, the surface tension and the curvature elastic constant of bilayers that form vesicles can be evaluated. If the vesicles contain hydrophilic solute molecules, thermal fluctuations of spherical vesicles depend on their osmotic pressure. Already at relatively low solute concentrations, the appearance of an osmotic pressure leads to a strong depression of vesicle fluctuations. The adsorption of colloidal particles on surfaces of soft materials causes elastic distortions. Biological membranes contain a large amount of embedded and adsorbed macromolecules, especially transmembrane and peripheral proteins consisting of large polypeptide chains folded in compact particles. Membrane distortions spread around each protein can superimpose and produce indirect forces between them. Similar effects are based on concentration fluctuations of the lipid components forming the membrane. Proteins as well as smaller peptides disturb the homogeneous distribution of the lipid mixture, facilitating a phase separation of the lipid components. Disturbances of the spatial lipid distribution can also be accompanied by an enhanced adsorption of water soluble peptides. This effect is amplified for nonideal mixtures, when the correlation length of concentration fluctuations is enlarged. Concentration fluctuations also produce forces between membrane proteins, which can enforce the aggregation of membrane bound proteins. Monte Carlo simulations are suitable for testing general concepts and theories on protein-membrane interactions.