Pressure-temperature phase diagram for shapes of vesicles: A coarse-grained molecular dynamics study

Abstract

Coarse-grained molecular dynamics simulations are performed to obtain the phase diagram for shapes of a vesicle with a variation in temperature and pressure difference across the membrane. Various interesting vesicle shapes are found, in particular, a series of shape transformations are observed for a vesicle with an initial spherical shape, which changes to a prolate shape, then an oblate shape, and then a stomatocyte shape, with either increasing temperature or decreasing pressure difference across the membrane.

Breakup of spherical vesicles caused by spontaneous curvature change

“…Between particles i and j, x x x i j = x x x j − x x x i , its distance is r i j = x x x i j and unit vectorx x x i j = x x x i j /r i j . The potential can be written as [15] …”

“…For the current model, a membrane is assumed to consist of single-layered particle with a diameter of r 0 , which is also the membrane thickness [15]. For particle i, 5 degrees of freedom are considered, that is, (x x x i , n n n i ), where x x x i is the position of particle i and n n n i is the surface normal vector at particle i, subject to the constraint that n n n i · n n n i = 1.…”

“…α is an energy penalty factor for the deviation of the membrane curvature from its spontaneous curvature. The governing equation of motion for particle i can be written as [15] …”

Morphology transition in lipid vesicles due to in-plane order and topological defects

“…In our coarse-grained simulations of membranes with tilt order, we generalize an earlier coarse-grained model for membranes without tilt order (25)(26)(27). We have recently used a similar model for membranes with internal nematic order (29).…”

“…For this purpose we superimpose a tilt orientation degree of freedom onto a highly coarse-grained membrane model recently introduced by Li and coworkers (25)(26)(27), where the lipid bilayer is represented by a single layer of point particles, each representing a patch of membrane of order 20 nm 2 , and solvent is implicit. To generalize this model to describe tilted lipid membranes, we assign each particle two vector degrees of freedom: a vectorn representing the outward layer normal direction, as in Li's original model, and an additional vectorĉ representing the local tilt direction, projected in the plane of the membrane (Fig.…”