Abstract
We present a simple and highly adaptable method for simulating coarse-grained lipid membranes without explicit solvent. Lipids are represented by one head bead and two tail beads, with the interaction between tails being of key importance in stabilizing the fluid phase. Two such tail-tail potentials were tested, with the important feature in both cases being a variable range of attraction. We examined phase diagrams of this range versus temperature for both functional forms of the tail-tail attraction and found that a certain threshold attractive width was required to stabilize the fluid phase. Within the fluid-phase region we find that material properties such as area per lipid, orientational order, diffusion constant, interleaflet flip-flop rate, and bilayer stiffness all depend strongly and monotonically on the attractive width. For three particular values of the potential width we investigate the transition between gel and fluid phases via heating or cooling and find that this transition is discontinuous with considerable hysteresis. We also investigated the stretching of a bilayer to eventually form a pore and found excellent agreement with recent analytic theory.
Highlights
Lipid bilayers are among the most versatile of nature’s biomaterials
At the smallest scale detailed quantum atomistic simulations are necessary to study the transport of ions or water across the membrane interface [1, 2], whereas at the opposite end of the length scale spectrum, analytic theory [3] and dynamically triangulated lattice simulations [4] have been used to determine the shape behavior of whole vesicles under various conditions of pressure, volume and area, or even under hydrodynamic flow [5]
If one could simulate bilayer membranes without the need for explicit solvent, a vast increase in accessible length and time scales would result, yet despite the long and successful history of solvent free models in polymer physics, this approach has not yet been widely adopted for lipid bilayer simulations. This is because the membrane case displays one additional complication: Unlike polymers, whose structure is at the outset determined by chemistry, lipids first have to physically self-assemble into a two-dimensional fluid bilayer
Summary
Lipid bilayers are among the most versatile of nature’s biomaterials. As the interface between the cell and its environment, or between organelles and the cytosol, they provide regulated transport of substances as small as protons to as large as entire cells. If one could simulate bilayer membranes without the need for explicit solvent, a vast increase in accessible length and time scales would result, yet despite the long and successful history of solvent free models in polymer physics, this approach has not yet been widely adopted for lipid bilayer simulations This is because the membrane case displays one additional complication: Unlike polymers, whose structure is at the outset determined by chemistry, lipids first have to physically self-assemble into a two-dimensional fluid bilayer. Concluding that simple pair potentials are insufficient, researchers have turned to the use of density dependent (multibody) interactions [6, 13, 14, 16, 17], angular dependent potentials [18] or highly tuned sets of Lennard-Jones like potentials [19] to stabilize the fluid bilayer phase without solvent (see Brannigan et al for a recent review [20]) Each of these approaches suffers from one or more significant drawbacks. V we study the stretching and rupture of a bilayer sheet and find near perfect agreement with a simple theoretical model developed by Farago [19] and Tolpekina et al [26] as well as with experimental data
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