Session: Physical Chemistry Poster:

Phospholipid bilayers are liquid-crystalline materials whose intermolecular interactions at mesoscopic length scales play key roles in the emergence of membrane physical properties. Here we investigated how such interactions are modulated by membrane hydration and cholesterol by solid-state ²H NMR spectroscopy and neutron spin-echo (NSE) spectroscopy combined with molecular simulations. When solid-state NMR spectroscopy is used, the average structure is manifested by the spectral lineshapes in terms of the coupling tensors due to quadrupolar or magnetic dipolar interactions. The residual quadrupolar couplings (RQCs) observed with ²H NMR spectroscopy directly yield the segmental order parameters (S_CD) for the individual acyl segments, providing average membrane properties such as the bilayer thickness, interfacial area per lipid, monolayer spontaneous curvature, and bending rigidity. Membrane dehydration and greater cholesterol mole fraction result in larger RQCs indicative of membrane thickening. Our nuclear magnetic relaxation experiments further show that elastic membrane deformations are revealed as collective order-director fluctuations (ODF). At mesoscopic length scales, the thermal fluctuations correspond to membrane mechanical properties in accord with the fluctuation-dissipation theorem, consistent with the results of NSE spectroscopy. Model-free interpretation of the functional dependence of NMR spin-lattice relaxation rates (R_1Z) on segmental order parameters (square-law) further reveals emergent material properties in the liquid-crystalline state. The R_1Z relaxation dispersion and temperature dependence of square-law plots indicate a nematic-like membrane hydrocarbon core. Solid-state ²H NMR relaxation studies demonstrate that both dehydration and increased cholesterol lead to decreased slopes of square-law plots, indicating an increase in membrane bending rigidity, in agreement with our neutron spin-echo measurements and molecular dynamics (MD) simulations. Molecular simulations yield crucial insights into the correspondence of the solid-state NMR and neutron spin echo results, and provide a unified interpretation of the square-law functional dependence observed when NMR experiments are conducted in a relaxometry mode.

Michael Brown

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