Studies of water diffusion in the vicinity of single-supported lipid membranes
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The insertion and function of membrane-embedded proteins is one of the most fundamental challenges facing biological physics today. At its core, addressing these phenomena requires an understanding of the interaction of proteins with the lipid bilayer and its associated water molecules. As such, the study of water dynamics and structure near model lipid bilayers can provide foundational knowledge upon which more detailed understanding of these core issues may be developed. Previous quasielastic neutron scattering measurements on lipid membranes have used samples of large stacks of membranes with an unknown amount of water between layers. This geometry complicates interpretation and renders comparison to molecular dynamics simulations difficult. Instead, this work investigates water dynamics on single-supported bilayers of the model charge-neutral lipid DMPC (dimyristoyl-sn-glycero-3-phosphocholine) and its anionic analogue DMPG (dimyristoyl-sn-glycero- 3-phosphoglycerol). Single bilayers can be more directly compared to molecular dynamics simulations, can be interrogated with Atomic Force Microscopy, and avoid the uncertainty in quantifying the amount of water in samples. A new method for producing the anionic bilayers is developed, which is a variant of the vesicle fusion method. Atomic Force Microscopy is used to characterize the quality of both DMPC and DMPG membranes supported on SiO?��-coated silicon substrates. Measuring the bilayer thickness as a function of temperature reveals that the gel-to-fluid phase transition is found to be shifted to significantly higher temperatures for adsorbed lipid bilayers in air compared to free vesicles in solution. The temperature-dependent quasielastic spectra from hydrated DMPC bilayers reveal three types of membrane-associated water. First, a large amount of water diffuses similarly to bulk supercooled water and freezes at 265 K. Second, a smaller amount of water closer to the membrane diffuses more slowly than bulk supercooled water at the same temperature and free
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