(822f) How Do Charge Density and Roughness of Surface Affect Formation of Supported Lipid Bilayers: A Coarse-Grained Molecular Dynamics Simulation

Dispersion of uni-lamellar vesicles to attractive surface is widely used to create supported lipid bilayers (SLBs), which are frequently used to embed membrane proteins for biosensors and novel artificial membranes for water or ion transport. Although vesicle spread and fusion is the most convenient and frequently used way to prepare SLBs with different composition and supporting surface in experiments, the mechanism of their formation and its behavior on surfaces with different properties are not well understood. To solve above-mentioned problem, we performed coarse-grained molecular dynamics simulation to study vesicle spreading mechanism on charged surface with special emphasis on charge type, charge density, surface roughness and also the existence of membrane protein. The liposome is composed of 677 DPPC molecules (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) and 200 negatively charged DPPG molecules (1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol). The model membrane protein is AqpZ, a kind of aquaporin protein from bacteria, and the charged surface is modeled by a plane of fixed particles with charges and some degree of roughness. It is shown that the liposome is only adsorbed on the surface and does not spread to from SLBs when the solid surface carries negative or zero charge. Once the surface is positively charged, the electrical interaction between the surface and the liposome enabled the liposome to overcome the bending energy barrier in the spreading process, which leads to the spread of the liposome. It is also shown that the lipids in the inner layer display several peaks in the density profile in the normal direction of the solid surface, indicating more multilayer structure forms once lipid adsorbed on the charge surface. Once the surface charge density increases, the SLBs become thinner, and more DPPG lipids appear in the inner layer. When AqpZ is embedded into liposome, the spreading process becomes more complex. It is shown that at low surface charge density, the liposome forms micelle like structure with AqpZ in it or non-flat lipid bilayer with AqpZ protuberance, rather than uniformed flat lipid bilayer with embedded protein. Only when surface possesses enough charge density, i.e., the interaction between the lipids and the surface is large enough, does lipid bilayer form flatly and  AqpZ embedded in it with correct orientation. This is important for keeping fluid mosaic structure of SLBs, which is critical for functional membrane protein. Above simulation results would help us to design suitable surface for spreading lipid bilayer with membrane protein.