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Discerning the Structure and Hydration of Unilamellar and Multilamellar Phosphatidylserine Bilayers Using Molecular Dynamics

Dipalmitoylphosphatidylserine (DPPS) is an anionic lipid that serves as a major constituent of the eukaryotic membrane. The experimental research of PS membrane models often prefers multilamellar (ML) systems due to higher stability, while computational studies model unilamellar (UL) bilayers. Here, classical molecular dynamics was employed to simulate both ML and UL DPPS systems in gel and fluid phase. Lipid bilayer containing 122 DPPS and 6 dipalmitoylphosphatidylglycerol (DPPG) molecules was simulated using GROMACS software. DPPG was added to be consistent with previous experimental work. Both systems employed periodic boundary conditions. UL systems contained a thick layer of water on both sides of the bilayer, thus preventing the interaction of the membrane with its periodic images. ML systems contained only a 2 nm layer of water between the images, mimicking the thickness of interbilayer water observed experimentally. The simulations were run for 150 ns. Structural parameters of the bilayer, hydration and the effects of water on both interfaces were evaluated. Membrane thickness, lipid ordering and lateral lipid diffusion indicated slightly higher packing and rigidity of ML systems. In both cases the bilayer was fully hydrated and water formed extensive hydrogen bonding network with lipid headgroups and the glycerol backbone. Lipids influenced water organization in the same way within the first hydration shell, however in ML systems the structuration of water persisted throughout the interlamellar space. Coupled with the 2-fold reduction in water diffusion coefficients between UL and ML, this leads to the conclusion that water in UL simulations behaves predominantly as bulk water, while in ML simulations the majority of waters behave as interfacial water. These findings will be combined with our experimental research in order to provide insight into differing thermotropic and molecular properties of UL and ML liposomes.

lipid bilayers ; unilamellar ; multilamellar ; molecular dynamics

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