Tunable generic model for fluid bilayer membranes

Abstract

We present a model for the efficient simulation of generic bilayer membranes. Individual lipids are represented by one head bead and two tail beads. By means of simple pair potentials these robustly self-assemble to a fluid bilayer state over a wide range of parameters, without the need for an explicit solvent. The model shows the expected elastic behavior on large length scales, and its physical properties ͑e.g., fluidity or bending stiffness͒ can be widely tuned via a single parameter. In particular, bending rigidities in the experimentally relevant range are obtained, at least within 3 -30k B T. The model is naturally suited to study many physical topics, including self-assembly, fusion, bilayer melting, lipid mixtures, rafts, and protein-bilayer interactions. Lipid molecules in aqueous solution spontaneously assemble into bilayer membranes. In biological systems, such membranes are involved in tasks over an extraordinary range of length scales, from transport of water and ions at the scale of nanometers, up to phagocytosis, amoebal motion and cell budding at the scale of microns ͓1͔. Computer simulations designed to understand some aspects of this structural and functional range must be tailored to the specific length and timescales involved. Techniques that probe both the smallest ͓2͔ and largest ͓3͔ of these length and time scales are now well established; however, accessing intermediate regimes has proven far more difficult, and it is only recently that significant progress has been made in this regard. The need for a comprehensive suite of techniques to study lipid bilayers at the mesoscale is highlighted by the sheer number of relevant problems in this regime, which include viral budding, raft formation, fusion, phase separation of multicomponent systems, and protein sorting during vesiculation.Most existing approaches to mesoscale simulation employ coarse grained lipids and require explicit solvent particles to stabilize the bilayer. This strategy is convenient and natural, yet it comes at a high price: Already for small flat systems the solvent accounts for most of the computational effort, but the problem gets significantly worse when three dimensional objects such as vesicles are to be simulated. Membrane and solvent play the role of surface and bulk, respectively, hence, the solvent degrees of freedom vastly outnumber the lipids even for rather modest sized vesicles. An obvious solution to this problem is to replace the solvent by effective lipid interactions. Given the great success of this approach in polymer physics it is perhaps surprising that solvent-free bilayer simulations have so far failed to find widespread acceptance. The problem appears to be that naive choices for interparticle potentials ͑e.g., Lennard-Jones͒ do not lead to a fluid bilayer phase but only to ordered "solid" bilayers at low temperature and low density phases at high temperature. Many attempts to obtain a broadly stable fluid phase have been made with varying degrees of success. So far, however, none of these has result...

Electrostatic Screening and Charge Correlation Effects in Micellization of Ionic Surfactants

“…32 Implicitsolvent models are helpful in reducing the computational cost of simulations. [33][34][35][36][37][38][39][40][41][42] In comparison with atomistic simulations, one can save 2 orders of magnitude in computation time using implicit-solvent models, 42 thus enabling self-assembly studies on longer time and length scales. We recently developed such an implicit-solvent model that is capable of reproducing the experimental critical micelle concentration (cmc) and the aggregate structures of ionic surfactants, such as sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium chloride (DTAC).…”

An implicit solvent coarse-grained lipid model with correct stress profile

“…We also compute the bending modulus of the DPPC bilayer as described by Cooke and Deserno. 28,29 The lipid tail position is averaged and interpolated on a 16ϫ 16 grid, and the average power fluctuations are measured and compared to the continuum model 37,38 …”

“…However, in 2003 Farago 27 employed a simple three-site lipid model with six interaction types, and was able to obtain a fluid bilayer. Deserno and co-workers 28,29 investigated how a broad, attractive tail-tail potential could stabilize the fluid phase, presumably because of the entropic gain of the expanded minimum. Brannigan et al 30 developed a lipid model with an interface bead at the head-tail intersection whose interactions have a less-steep curvature and were also able to simulate the fluid phase and aspects of an all-atom stress profile.…”

Adaptive resolution molecular-dynamics simulation: Changing the degrees of freedom on the fly

“…One first way to tackle this, was to reduce the number of de-grees of freedom by a systematic coarse-graining, which retains only those degrees of freedom that are relevant for the particular property of interest. Examples of molecular systems where the coarsegraining approach has been used with much success are fluids [9], lipid bilayers [10,11,12,13,14], and polymer systems [15,16,17,18].…”