Simulating Lipid Membrane Phase Separation and Morphology

Katsuyo Thornton

L.H. and F.E. Van Vlack Professor

kthorn@umich.edu

2022 HH Dow

T: (734) 615-1498

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Lipid membranes are an interesting system: since the lipid molecules composing the bilayer are held together only with hydrophobic interactions rather than chemical bonds, they form a liquid-like structure constrained in two dimensions. Experimentally, it has been found that lipid bilayers and liposomes made from certain mixtures of lipid molecules undergo phase separation below a certain temperature. The phase separation can then induce shape changes in the membrane, since the mechanical properties of the multiple phases can differ. These mechanical properties include bending rigidity, which results from the packing of different lipids, and spontaneous curvature, which also results from packing but is closely related to the shape of the specific lipid molecules. Our group (Chloe Funkhouser, doctoral student in Biomedical Engineering; Dr. Francisco Solis, Associate Professor in the Division of Mathematical and Natural Sciences at Arizona State University; and Dr. Katsuyo Thornton) investigates how compositional changes (e.g. phase separation) affect the shape of the membrane, and also how shape changes can affect the composition. A phase-field method is used to model phase separation, and local composition is coupled with local geometry. Simulations of nearly planar monolayer membranes have shown that striped morphologies are difficult to form from random mixtures even when they are the lowest-energy thermodynamic configuraton. Simulations of bilayer membranes, with additions to the model to account for interactions between the leaflets of the bilayer, have allowed us to produce morphological phase diagrams at different coupling strengths. We are currently applying our spherical membrane liposome simulation model to investigate how the mechanical coupling in the model affects compositional evolution.