Solid-state deuterium NMR spectroscopy is a powerful method in colloid and surface chemistry for addressing how material properties emerge from atomistic level interactions of surfactants and lipid membranes. Here we show that solid-state 2H NMR relaxation provides atomistically resolved information about the structure and dynamics of flexible phospholipids in the liquid-crystalline state. For lamellar mesophases, the average structure is manifested by the segmental order parameters (SCD) of the lipid molecules. Material properties are obtained from measurements of static lipid deformations, and thermal motions in liquid-crystalline membranes are directly related to membrane mechanical properties via the fluctuation-dissipation theorem. Because of the temporal range of solid-state 2H NMR relaxation methods, the hierarchical dynamics of liquid-crystalline membranes have become accessible. Model-free interpretation of the functional dependence of spin-lattice relaxation rates (R1Z) on the SCD order parameters informs the composite lipid dynamics in the liquid-disordered (ld) state. Our approach reveals that membrane elasticity involves bilayer deformations over a range of length scales, from the nanoscale down to the size of the flexible lipid segments. Elastic membrane deformations involve director fluctuations that are characterized by a square-law dependence of the nuclear spin relaxation rates on the segmental order parameters, and by their dependence on the magnetic field strength (Larmor frequency). We establish how NMR relaxation guides the continued development of atomistic and coarse-grained molecular mechanics force fields. Collective bilayer excitations are emergent over mesoscopic length scales that elucidate how membrane mechanics are governed by the lipid acyl chains, polar head groups, cholesterol, and proteins. Solid-state 2H NMR relaxation studies further reveal lipid-specific effects of cholesterol on the bending rigidities of both saturated (DMPC) and unsaturated (POPC and DOPC) bilayers in the liquid-disordered (ld) state in terms of their viscoelastic properties. Future conceptual advances and theoretical reductions will enhance our understanding of hierarchical biomembrane dynamics through a synergistic integration of NMR measurements, molecular simulations, and neutron scattering studies.

Elastic moduli and collective dynamics of phospholipids are revealed by solid-state 2H NMR spectroscopy

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

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