This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Biological membranes are structures composed of multiple lipid species and proteins. The lipids are bound only through hydrophobic interactions, creating a liquid-like structure. The plasma membrane, a lipid bilayer membrane surrounding all mammalian cells, is not homogeneous, but rather contains domains termed rafts, defined as regions enriched with cholesterol and saturated lipids. Understanding how and why these rafts form is of great importance to cell biologists and immunologists, since they are involved in many important cell functions and processes including endocytosis, cell adhesion, signaling, protein organization, lipid regulation, and infection by pathogens. These raft structures also show great potential for technological applications, especially in connection with biosensors and drug delivery systems. We examine phase separation (lipid raft formation) and morphological evolution of multicomponent lipid bilayer membranes. The model applies to membranes with planar and spherical background geometries, simulating a nearly planar portion of a membrane or an entire vesicle, respectively. The model treats the individual composition of each bilayer leaflet, which determines the spontaneous curvature. The compositions and shape of the membrane are coupled with a modified Helfrich free energy, which includes coupling between the leaflets compositions. The compositional evolution is modeled using a phase-field method and is described by a Cahn-Hilliard-type equation, while the shape changes are described by relaxation dynamics. For nearly planar bilayer systems we construct a phase diagram of equilibrium morphological phases in the composition space for a few values of the strength of the leaflet coupling. For vesicles modeled using a spherical background, our investigations have focused on how the dynamics are affected by spontaneous curvature effects.
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