Vesicle Deformation by Draining: Geometrical and Topological Shape Changes

Abstract

A variety of factors, including changes in temperature or osmotic pressure, can trigger morphological transitions of vesicles. Upon osmotic upshift, water diffuses across the membrane in response to the osmotic difference, resulting in a decreased vesicle volume to membrane area ratio and, consequently, a different shape. In this paper, we study the vesicle deformations on osmotic deflation using coarse grained molecular dynamics simulations. Simple deflation of a spontaneously formed spherical vesicle results in oblate ellipsoid and discous vesicles. However, when the hydration of the lipids in the outer membrane leaflet is increased, which can be the result of a changed pH or ion concentration, prolate ellipsoid, pear-shaped and budded vesicles are formed. Under certain conditions the deflation even results in vesicle fission. The simulations also show that vesicles formed by a bilayer to vesicle transition are, although spontaneously formed, not immediately stress-free. Instead, the membrane is stretched during the final stage of the transition and only reaches equilibrium once the excess interior water has diffused across the membrane. This suggests the presence of residual membrane stress immediately after vesicle closure in experimental vesicle formation and is especially important for MD simulations of vesicles where the time scale to reach equilibrium is out of reach.

Surface Density-Induced Pleating of a Lipid Monolayer Drives Nascent High-Density Lipoprotein Assembly

“…The total number of CG beads reached a maximum of approximately 520,000 (in the system with 7 apoA-I dimers). SUV were created by CGMD of bilayer discs containing 2422 POPC or DMPC (Markvoort et al, 2009). …”

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

“…The CGMD model used here is coarser than that used in previous studies. 12,[18][19][20] In the present model, a membrane particle with a diameter of membrane thickness and described by five degrees of freedom, actually represents a number of lipid molecules. In addition, this model eliminates the explicit consideration of solvent molecules, and their effect is considered by the effective particle interaction potential.…”

“…Since the meso-or macroscopic properties of membranes cannot possibly depend on all the details of the atomic description, coarse-grain atomistic models have been used to study the physical properties of membranes, including formation of membrane structures, 8 membrane elasticity, [8][9][10][11][12][13][14][15][16][17][18] thermal fluctuation, [8][9][10]17 nanoparticle endocytosis, 10,16 composition segregation, 12,[14][15][16] and topological shape changes. [18][19][20] A vesicle may change its shape, volume, or surface area, due to the change in properties of its membrane and/or the presence of external loadings. 21 A phase diagram for vesicle shape transformation at different membrane properties and external loading conditions reveals important thermodynamic behavior of vesicle.…”

“…Different phases of vesicle shapes, ranging from stomatocytes, pears, prolates, and oblates, were obtained. 19,20,23 However, the phase diagram for shape transformation of a vesicle involving the variation of temperature and the pressure difference across the membrane is still not available. Here coarse-grained molecular dynamics ͑CGMD͒ simulations are performed to obtain the phase diagram for vesicle shape transformation at various temperatures and cross-membrane pressure differences.…”

Coarse-grained simulations of poly(propylene imine) dendrimers in solution

“…This coarse-grained model for PPI is illustrated in Figure 1. Its 3 parameterization was based on a combination of matching thermodynamic data (like for our earlier lipid model [18][19][20] ) and a Boltzmann inversion scheme on bond and angle distributions of virtual coarse-grained sites accumulated from atomistic simulations of G5 PPIUA. Those simulations as well as all simulations described in this paper were performed using our in-house developed MD platform PumMa 23 .…”