Abstract
Biomimetic and biological membranes consist of molecular bilayers with two leaflets that are typically exposed to different aqueous solutions. We consider solutions of "particles" that experience effectively repulsive interactions with these membranes and form depletion layers in front of the membrane leaflets. The particles considered here are water-soluble, have a size between a few angstrom and a few nanometers as well as a rigid, more or less globular shape, and do neither adsorb onto the membranes nor permeate these membranes. Examples are provided by ions, small sugar molecules, globular proteins, or inorganic nanoparticles with a hydrophilic surface. We first study depletion layers in a hard-core system based on ideal particle solutions as well as hard-wall interactions between these particles and the membrane. For this system, we obtain exact expressions for the coverages and tensions of the two leaflets as well as for the spontaneous curvature of the bilayer membrane. All of these quantities depend linearly on the particle concentrations. The exact results for the hard-core system also show that the spontaneous curvature can be directly deduced from the planar membrane geometry. Our results for the hard-core system apply both to ions and solutes that are small compared to the membrane thickness and to nanoparticles with a size that is comparable to the membrane thickness, provided the particle solutions are sufficiently dilute. We then corroborate the different relationships found for the hard-core system by extensive simulations of a soft-core particle system using dissipative particle dynamics. The simulations confirm the linear relationships obtained for the hard-core system. Both our analytical and our simulation results show that the spontaneous curvature induced by a single particle species can be quite large. When one leaflet of the membrane is exposed, e.g., to a 100 mM solution of glucose, a lipid bilayer can acquire a spontaneous curvature of ±1/(270 nm). Our theoretical results can be scrutinized by systematic experimental studies using a large variety of different types of particles.
Highlights
Biological and biomimetic membranes consist of bilayers with two leaflets of lipid molecules
Our results for the hard-core system apply both to ions and solutes that are small compared to the membrane thickness and to nanoparticles with a size that is comparable to the membrane thickness, provided the particle solutions are sufficiently dilute
III, we describe our computational method based on Dissipative Particle Dynamics (DPD)
Summary
Biological and biomimetic membranes consist of bilayers with two leaflets of lipid molecules. The simulations extend our recent study in which we determined the membrane curvature arising from asymmetric adsorption layers.. As a result of the effectively attractive interactions, the particle mixture can undergo phase separation.. As a result of the effectively attractive interactions, the particle mixture can undergo phase separation.5 These depletion effects have been studied in some detail, both theoretically and experimentally, see the review in Ref. 12 The exclusion of the smaller particles by the larger ones leads to an effective attraction between the larger particles because the smaller particles gain translational entropy when the larger ones move closer together. As a result of the effectively attractive interactions, the particle mixture can undergo phase separation. These depletion effects have been studied in some detail, both theoretically and experimentally, see the review in Ref. 12
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