Simulation of fluid/gel phase equilibrium in lipid vesicles

Area of Science:

• Biophysics
• Computational Chemistry
• Materials Science

Background:

• Lipid bilayers are fundamental to cell membranes.
• Understanding phase transitions in nanoscale lipid systems is crucial for drug delivery and biomaterials.
• Finite size effects can significantly alter the properties of lipid vesicles.

Purpose of the Study:

• To investigate the impact of vesicle size on the gel/fluid phase transition of dipalmitoylphosphatidylcholine (DPPC) lipid vesicles.
• To explore the role of curvature in finite size effects on lipid phase behavior.
• To characterize the structural changes and transition pathways in nanoscale lipid systems.

Main Methods:

• Coarse-grained molecular dynamics simulations using the DRY-MARTINI force field.
• Statistical Temperature Molecular Dynamics (STMD) for enhanced sampling of phase transitions.
• Analysis of phase transitions via the energy-dependence of statistical temperature, T_S(U).

Main Results:

• The transition temperature decreases with decreasing vesicle diameter (10 nm, 20 nm, 40 nm).
• The phase transition shifts from first order to borderline first-second order for smaller vesicles.
• Curvature-induced disruption of gel packing leads to inhomogeneous states with faceted gel patches and fluid seams.

Conclusions:

• Finite size effects, primarily driven by curvature, significantly alter lipid vesicle phase transitions.
• Nanoscale lipid systems exhibit unique phase behaviors distinct from the thermodynamic limit.
• A simple physical model explains phase transitions in finite lipid systems, with distinct inner and outer layer structures observed at high curvature.