How solid substrates affect a lipid bilayer: A molecular dynamics study

We use Molecular Dynamics (MD) simulations to investigate how solid substrates interact with a dipalmitoylphosphatidylcholine (DPPC) lipid bilayer and how substrates affect the lipid membrane structurally and dynamically. Supported lipid bilayers are extensively used in experiments as a simplified model for cell membranes. Few simulations of supported bilayers have been performed.We use the MARTINI coarse-grained model to study the supported bilayer computationally. When a lipid bilayer is deposited on the solid substrate, the undulation of the proximal leaflet is suppressed to a higher extent than that of the distal leaflet. Properties of the bilayer including density profiles and lateral mobility exhibit heterogeneity. The proximal leaflet is ordered under the influence of a smooth surface, while the distal leaflet remains the similar to the leaflets in a free-standing bilayer.The lateral pressure profile of the supported bilayer is also different from that of a free-standing bilayer. Membrane proteins undergo undesired conformational changes due to the change in the lateral pressure profile. We study in particular the high local pressure in the proximal leaflet and estimate the interfacial energy associated with it. The interfacial energy falls in the range of activation energy barriers of the large mechanosensative channel (MscL) and may contribute to the opening of MscL without external pressure.Experiencing the strong inwards tension, the proximal leaflet prefers a smaller area per molecule than the distal leaflet and free-standing bilayers. We employ the umbrella sampling technique to calculate the potential of mean force (PMF) of lipid translocation. The free energy difference given by the PMF of various supported bilayer systems indicate that the proximal leaflet needs more lipids than the distal leaflet. The equilibrium density imbalance between two leaflets in the supported bilayer is roughly 4-7%. In addition, the equilibrium area per molecule of the proximal leaflet is estimated to be around 58 A2, about 4 A2 smaller than that of the freestanding bilayer. The lipid flip-flop rate in a supported bilayer with the same number of lipids in both leaflets is higher than in the free-standing bilayer, because the energy barrier of flip-flop is lowered by a few kBT.