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Systematic multi-scale model for phospholipids Homogeneous bubble nucleation
Pore nucleation in bio-membrane Top-down Coarse-grained model for biomembranes
Novel advanced sampling techniques and its applications Perturbation-theory analysis on interfacal free energy
Liquid-vapor interphase transport Liquid-vapor interface properties

  • Systematically Coarse-Grained Solvent-Free (CG SF) Model for Phospholipids
  • J Phys Chem B, 2010, 114: 11207

    We presented an implicit solvent coarse-grained (CG) model for quantitative simulations of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayers. The use of implicit solvent enables membrane simulations on large length- and time-scales at moderate computational expense. Despite an improved computational efficiency, the model preserves chemical specificity and quantitative accuracy. The bonded and nonbonded interactions together with the effective cohesion mimicking the hydrophobic effect were systematically tuned by matching structural and mechanical properties from experiments and all-atom bilayer simulations, such as saturated area per lipid, radial distribution functions, density and pressure profiles across the bilayer, P2 order, etc. The CG lipid model is shown to self-assemble into a bilayer starting from a random dispersion. Its elastic properties, such as bending and stretching modulus, are semi-quantitatively consistent with experiments. The effects of reduced molecular friction and more efficient integration combine to an overall speedup of three to four orders of magnitude compared to all-atom bilayer simulations. Our CG lipid model is especially useful for studies of large-scale phenomena in membranes which nevertheless require a fair description of chemical specificity, e.g. membrane patches interacting with movable and transformable membrane proteins and peptides.

  • Transferability of the Systematic CG SF Force Field for Phospholipids
  • New J Phys, 2010, 12: 095004

    We study lipid and phase transferability of our recently developed systematically coarse-grained solvent-free membrane model. The force field was explicitly parametrized to describe a fluid POPC bilayer at 310 K with correct structure and area per lipid, while gaining at least three orders of magnitude in computational efficiency ( J Phys Chem B, 2010, 114: 11207 ). Here we show that exchanging CG tails, without any subsequent re-parameterization, creates reliable models of DOPC and DPPC lipids in terms of structure and area per lipid. Furthermore, all CG lipids undergo a liquid-gel transition upon cooling, with characteristics as observed in experiments and all-atom simulations during phase transformation. These studies suggest a promising transferability of our force field parameters to different lipid species and thermodynamic state-points, properties that are prerequisite for even more complex systems, such as mixtures.

  • Homogeneous Bubble Nucleation Driven by Local Hot Spots
  • J Phys Chem B, 2009, 113: 3776

    We report a Molecular Dynamics study of homogenous bubble nucleation in a Lennard-Jones fluid. The rate of bubble nucleation is estimated using forward-flux sampling (FFS). FFS is well suited to simulate nucleation processes because it does not assume that the nucleation process is slow compared to the time it takes all other degrees of freedom to equilibrate. The latter assumption is implicit in the Umbrella Sampling scheme.

    We find that cavitation starts with compact bubbles rather than with ramified structures as had been suggested by Shen and Debenedetti (J Chem Phys, 111:3581, 1999). Our estimate of the bubble-nucleation rate is higher than predicted on the basis of Classical Nucleation Theory (CNT). Our simulations show that local temperature fluctuations correlate strongly with subsequent bubble formation - this mechanism is not taken into account in CNT.

  • Pore Nucleation in Mechanically Stretched Bilayer Membranes
  • J Chem Phys, 2005, 123: 154701

    We report a computer-simulation study of the free-energy barrier for the nucleation of pores in the bilayer membrane under constant stretching lateral pressure. We find that incipient pores are hydrophobic but as the lateral size of the pore nucleus becomes comparable with the molecular length, the pore becomes hydrophilic. In agreement with previous investigations, we find that the dynamical process of growth and closure of hydrophilic pores is controlled by the competition between the surface tension of the membrane and the line tension associated with the rim of the pore. We estimate the line tension of a hydrophilic pore from the shape of the computed free-energy barriers. The line tension thus computed is in a good agreement with available experimental data. We also estimate the line tension of hydrophobic pores at both macroscopic and microscopic levels. The comparison of line tensions at these two different levels indicates that the "microscopic" line tension should be carefully distinguished from the "macroscopic" effiective line tension used in the theoretical analysis of pore nucleation. The overall shape of the free-energy barrier for pore nucleation shows no indication for the existence of a metastable intermediate during pore nucleation.

  • A Solvent-Free Coarse-Grained Model for flexible amphiphilic bilayers: Off-Lattice and Three-Beads Monte Carlo Simulations
  • J Chem Phys, 2005, 122: 234711

    We present a simple, implicit-solvent model for fluid bilayer membranes. The model was designed to reproduce the elastic properties of real bilayer membranes. For this model, we observed the solid-fluid transition and studied the in-plane diffusivity of the fluid phase. As a test, we compute the elastic-bending and area-compressing moduli of fluid bilayer membranes.We find that the computed elastic properties are consistent with the available experimental data.

  • Perturbation-Theory Estimates for the Liquid-Solid and the Liquid-Vapor Interfacial Free Energies of Lennard-Jones Systems
  • Molecular Simulation, 2007, 33: 1023.

    The most naive perturbation method to estimate interfacial free energies is based on the assumption that the interface between coexisting phases is infinitely sharp. Although this approximation does not yield particularly accurate estimates for the liquid-vapor surface tension, we find that it works surprisingly well for the interface between a dense liquid and a solid. As an illustration we estimate the liquid-solid interfacial free energy of a Lennard-Jones (LJ) system with truncated and shifted interactions and compare the results with numerical data that have been reported in the literature.

    We find that the agreement between theory and simulation is excellent. In contrast, if we apply the same procedure to estimate the variation of the liquid-vapor surface tension, for different variants of the LJ potential (truncated/shifted/force-shifted), we find that the agreement with the available simulation data is, at best, fair. The present method makes it possible to obtain quick and easy estimate of the effect on the surface free energy of different potential-truncation schemes used in computer simulations.

  • Molecular Dynamics Study on the Liquid-Vapor Interphase Transport
  • Microscale Thermophysical Engineering, 2003, 7: 275

    The evaporation and condensation processes of argon at the liquid-vapor interface are investigated with molecular dynamics simulations. By introducing a novel statistical method, namely, the characteristic time method, the condensed then re-evaporated molecules are successfully distinguished from the reflected molecules according to their different characteristic times, which are estimated based on the statistic during simulation. The kinetic characteristics of the condensed, reflected, and evaporated molecules are separately studied and compared. The results show that the high kinetic energy component normal to the interface is an important character of the interphase transport process.

  • Molecular Dynamics Study on the Liquid-Vapor Interfacial Profiles
  • Fluid Phase Equilibria, 2001, 183-184: 321

    Molecular dynamics simulations are carried out to study the thermodynamic properties in the liquid-vapo coexistent systems with liquid-vapor interfaces. The interactions between the particles are modeled with a truncated Lennard-Jones pair potential. The density profile, the temperature profile and the pressure tensor are obtained at two different bulk temperatures. There exist a sharp peak and a small valley at the thin region outside the liquid-vapor interface in the local kinetic energy distribution across the interface. When the system temperature increases, the magnitude of the peak and the valley decreases. The non-equilibrium molecular kinetic energy distribution located at the thin region outside the liquid-vapor interface confirms that though the liquid and vapor phases are in thermodynamic equilibrium, the interface may not be in thermostatic equilibrium. This kind of molecular kinetic energy distribution may embody the behavior of energy transport between the liquid and the vapor phases.