Recent experiments by L. Hirst have shown that lipid DPPC vesicles becomes disordered in shape when quenched into the tilted gel phase. We propose that this shape evolution is driven by nucleation and motion of topological defects in membrane tilt, and explore this phenomenon using a coarse-grained simulation approach. Simulation studies of small vesicles produce equilibrium shapes comparable to those predicted by analytical theory (e.g. by Lubensky.) But in larger vesicles, simulations show that  defects can become trapped in deeply metastable states, leading to more disordered structures; we compare with Hirst's experimental findings. Next we extend the model to study morphology transitions in a lipid vesicle with nematic order at its surface, e.g. composed of lipids with a rod-shaped head group. By increasing the coupling between defects and membrane curvature, we predict a morphology transition from sphere to prolate ellipsoid. As coupling is further increased, pores nucleate and coalesce, producing a hollow tube. We discuss how our coarse-grained modeling techniques can be extended for simulation studies of a variety of lipid structures too large to model at the molecular scale.
* Work carried out with Jun Geng and JV Selinger; supported by NSF-DMR-0605889.