A SOLVENT-FREE COARSE-GRAINED MODEL FOR BIOLOGICAL AND BIOMIMETIC FLUID MEMBRANES
Open Access
- Author:
- Yuan, Hongyan
- Graduate Program:
- Engineering Science and Mechanics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 18, 2010
- Committee Members:
- Sulin Zhang, Dissertation Advisor/Co-Advisor
Sulin Zhang, Committee Chair/Co-Chair
Peter J Butler, Committee Member
Long Qing Chen, Committee Member
Bruce Gluckman, Committee Member
Bernhard R Tittmann, Committee Member - Keywords:
- lipid bilayer
fluid membranes
bending rigidity
coarse-grained
solvent-free
membrane models
molecular dynamics simulations
anisotropic interparticle interaction
liquid crystal - Abstract:
- Mechanics of biological membranes is involved in numerous intriguing biophysical and biological cellular phenomena of different length scales. On the length scale much larger than the membrane thickness, fluid bilayer membranes or biomimetic membranes, from the mechanics viewpoint, can be simplified as “thin fluid shells” with their mechanical behaviors dictated by only a few effective mechanical properties such as bilayer membrane bending rigidity, area compression modulus, in-plane viscosity, line tension between two different components, and spontaneous curvature. Solving mechanics problems of the fluid membranes with complicated shapes or multi-components in static or dynamic conditions largely resorts to computer simulations. There are two approaches to establish a simulation model for fluid membranes at the large length scale. One is the numerical implementations of the continuum membrane models. The other is the highly coarse-grained inter-particle interaction-based membrane simulation models. One of the drawbacks of the former class of models is that extra or dedicated computational effort is needed to take into account the in-plane fluidity and topological changes. Conversely, the latter class of models can naturally capture the in-plane fluidity and viscosity, and simulate topological changes. On the other hand, the disadvantage of the inter-particle interaction-based models is that they suffer length scale limitations due to the small intrinsic particle size, which is not the case for the former class of models. In this study, we established a one-particle-thick fluid membrane model, where each particle represents a cluster of lipid molecules. The model features an inter-particle pair potential with the interaction strength weighed by the relative particle orientations. The model is solvent-free, and the orientation dependence of the inter-particle pair potential substitutes for the hydrophobic effect. Particles can robustly self-assemble into fluid membranes with experimentally relevant membrane properties such as bending rigidity. Three potential parameters separately and effectively control diffusivity, bending rigidity, and spontaneous curvature of the model membrane. The high level of coarse-graining and the efficiency of the model enable the studies of large-scale membrane problems that are typically not accessible by previous coarse-grained models. This model is well suited to study the mechanics of both homogeneous and heterogeneous fluid vesicles, such as morphology and shape changes in static or dynamic situations. In order for this solvent-free membrane model to be biologically or experimentally relevant in the fluid vesicle case where the vesicle volume is controlled by the osmotic pressure, the membrane model is extended to the fluid vesicle case by incorporating a volume-control algorithm based on an external potential associated with vesicle volume. The instantaneous volume of the vesicles is calculated via an efficient, accurate and robust local triangulation algorithm. The shape transformation pathways produced by the present model agree strikingly well with previous experimental data. Furthermore, we have studied very first the non-equilibrium behaviors of fluid vesicles under different volume-change rate. The results show that both the intermediate and equilibrium vesicle configurations depend on the volume-change rate, which manifests the viscous effect of the fluid membrane. Recently, the membrane-mediated repulsive interactions between liquid-ordered domains in model membranes are believed to play important role in stabilizing the finite domain size. Using the membrane model, we studied the domain growth dynamics on a binary fluid vesicle. The both normal and slow domain growth dynamics observed in the experiments was reproduced in our simulations. The results show that the repulsive interactions depend on domain size and domain curvature. As budded domains grow bigger, the repulsive energy becomes comparable to the thermal energy and thus set an energy barrier to slow down approaching and further coalescence of domains. The membrane model developed in this thesis research is mathematically and physically easy to follow, of the highest level of coarse-graining with the inter-particle interaction, and naturally includes the various key mechanics elements of fluid membranes such as in-plane fluidity and viscosity, out-of-plane bending rigidity, line tension, edge energy, topological changes, effective spontaneous curvature, thermal fluctuations. The model provides a computationally reliable and powerful alternative to experiments and continuum theories for the study of large-scale fluid membrane mechanics. In addition, the model itself provides a simple model system for 2D self-assembled membranes in the mesoscopic scale, and thus can help understand the fundamental aspects of the condensed soft matter physics of self-assembled membranes, similar to the Lennard-Jones potential for 3D condensed matter physics.