Computer Simulation of Solid-Supported Lipid Bilayers: Interfacial Structure and Adhesion Mechanism
Theoretical Chemistry: Electronic Structure, Dynamics, Simulation
Final Report Abstract
Solid-supported lipid bilayers (SLBs) are popular models of cell membranes. The advantage of SLBs is an enhanced stability coupled with lateral mobility of supported lipid molecules in aqueous medium. To elucidate the mechanism of bilayer-support adhesion, the structure of the interfacial region, and the nature of water-mediated forces operating in the system, we resort to Grand Canonical Monte Carlo (GCMC) simulations, with the model membrane represented by a dilauroylphosphatidylethanolamine (DLPE) bilayer. As model supports, we use muscovite mica and also a gold substrate functionalized with alkanethiol self-assembled monolayers (SAMs). For a hydrophilic COOH-terminated SAM, the major contribution to the bilayer-SAM adhesion energy is due to direct electrostatic and dispersion attractions, while the repulsive hydration contribution is substantially lower in magnitude. In the case of a hydrophobic CH3-terminated SAM, the calculated adhesion energy is negligibly small, in agreement with experimentally observed inability of this SAM to adhere to phospholipid bilayers. Most interesting results are obtained with the DLPE bilayer supported on mica. At large separations, the system shows a hydration repulsion typical of hydrophilic confining surfaces. With decreasing separation, the pressure suddenly drops to negative (attractive) values. The reason is the protrusion of some individual DLPE molecules towards the mica support and the subsequent formation of chemical bonds with the mica surface atoms. Since the bilayer-mica links are widely spaced, there remains a water interlayer between the mica support and the lipid bilayer. It is this interlayer that maintains the lateral mobility of the lipid bilayer.
