Phospholipids are examples of biological liquid crystals exhibiting long-range order in their molecular organization, with interactions at mesoscopic length scales playing a key role in the emergence of mechanical properties of lipid membranes. The average structure is obtained from solid-state 2H NMR lineshapes that yield segmental order parameters (SCD) for the individual acyl segments, which are related to the average membrane properties such as area/lipid and spontaneous curvature. On the other hand, thermal motions in liquid-crystalline membranes are obtained from spin-lattice relaxations as a function of the order parameter SCD. These collective dynamics are related to the membrane mechanical properties by the fluctuation-dissipation theorem. Model-free interpretation of the functional dependence of spin-lattice relaxation rates (R1Z) on the segmental SCD order parameters informs the liquid-crystalline material properties of the lipid bilayer. Variation of both average and dynamic properties with key compositional membrane parameters reveal how the membrane mechanical properties emerge from the intermolecular interactions. These compositional variables include the lipid polar head groups, acyl chains, and cholesterol, which are accessible at the atomistic level using solid-state 2H NMR spectroscopy. For instance, mixtures of DPPE and DPPC show bending elasticities that depend on the molar ratio of PE to PC head groups in the liquid-disordered (ld) state. For the acyl chains, the emergent bending energy follows the sequence DOPC &lt; POPC &lt; DMPC. Furthermore, solid-state 2H NMR studies of the effect of cholesterol on the bending rigidities of both saturated (DMPC) and unsaturated (POPC and DOPC) bilayers in the liquid-ordered (lo) state reveal phospholipid-specific effects on the membrane viscoelastic properties. The square-law functional dependence of the R1Z relaxation rates on the segmental SCD order parameters (Fermi's Golden Rule), indicates membrane stiffening with increasing cholesterol content. Studies of membranes with different lipids (e.g., POPC vs. DOPC) reveal that the cholesterol-induced stiffening depends on acyl chain unsaturation. Solid-state 2H NMR probes emergent bending energies of lipid bilayers on mesoscopic length scales, consistent with a flexible surface model (FSM) for the influences of the soft matter on membrane proteins such as ion channels, receptors, and transporters.

Solid-state deuterium NMR spectroscopy reveals emergent bending energies of lipid membranes

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.