A coarse-Grained Solvent-Free Model for Tethered Lipid Membrane Simulation

Abstract

In order to characterize biological lipid membranes and their constituents, a variety of experimental techniques, for instance fluorescence microscopy and X-ray/neutron reflection, require the bilayer systems to have a robust planar geometry. However, the adsorption onto a substrate can strongly affect some properties of the system, especially in the proximal leaflet. To alleviate artifacts such as the immobility of membrane-associated proteins, tethered bilayer systems have been developed. In these systems tethers consisting of short hydrophilic polymers elevate the bilayer from the substrate and also provide better control over the separation between the two.Different computational models for lipid memebranes with varying levels of resolution have been proposed in the past. However, only a few simulations have been dedicated to the supported systems, and none have studied the tethered bilayers. Here we present a mesoscopic coarse-grained model of tethered lipid membranes, which is based on an implicit-solvent three-bead lipid model [Cooke et. al., Phys. Rev. E 72, 011506 (2005)]. Each tether molecule consists of 1) a hydrophobic part that can anchor into lipid leaflets and share the properties of the lipids, 2) a hydrophilic chain mimicking polyethelene glycol, and 3) a headgroup bead which covalently attaches to the substrate. Under suitable conditions, these fixed tether molecules and free random lipids self-assemble into a flat tethered bilayer. Different physical observables, such as the fluctuation spectrum, the area density of lipids, and diffusion coefficients in proximal/distal leaflets, are measured. The results are compared to free and solid-supported membranes, providing important insights into the generic properties of this system.