Solid-state ² H NMR studies of water- mediated lipid membrane deformation

The application of solid-state ² H nuclear magnetic resonance (NMR) spectroscopy gives a powerful approach for investigating hydration-mediated effects on lipid bilayer structure and dynamics. The extent to which lipid bilayers are deformed by dehydration stress is inherent to understanding how lipid-protein interactions affect biomembrane functioning. For liquid-crystalline membranes, the average structure is manifested by the segmental order parameters (S _CD ) of the lipids. Structural quantities, such as the area per lipid and volumetric bilayer thickness, are obtained by a mean-torque analysis of ² H NMR order parameters. Removal of water in the liquid-crystalline state gives a reduction of the mean area per lipid, together with a corresponding increase in volumetric bilayer thickness. Measurements of order parameters versus osmotic pressure yield the elastic area compressibility modulus and the corresponding bilayer thickness at an atomistic level. Furthermore, solid-state ² H NMR relaxation rates of lipid bilayers at varying hydration levels afford new insights into the role of water in membrane structural dynamics and viscoelastic properties. Model-free interpretation of spinlattice (R _1Z ) and transverse (R ^QE ₂ ) relaxation rates suggests that collective chain motions described as order-director fluctuations dominantly contribute to the relaxation. In a continuum picture, elastic deformations in such materials are collective hydrodynamic phenomena with motional time scales spanning many decades (picoseconds to seconds). The dynamic processes mainly affecting the spin-spin relaxation have characteristic time scales much longer than those contributing to spin-lattice relaxation. Such studies probe membrane interactions involving collective bilayer undulations, order-director fluctuations, and lipid molecular protrusions, giving a unique source of information about intermolecular forces pertinent to biomembrane structure and function.