Concepts of solid-state NMR spectroscopy and applications to fluid membranes are reviewed in this paper. Membrane lipids with 2H-labeled acyl chains or polar head groups are studied using 2H NMR to yield knowledge of their atomistic structures in relation to equilibrium properties. This review demonstrates the principles and applications of solid-state NMR by unifying dipolar and quadrupolar interactions and highlights the unique features offered by solid-state 2H NMR with experimental illustrations. For randomly oriented multilamellar lipids or aligned membranes, solid-state 2H NMR enables direct measurement of residual quadrupolar couplings (RQCs) due to individual C-2H-labeled segments. The distribution of RQC values gives nearly complete profiles of the segmental order parameters SCD(i) as a function of acyl segment position (i). Alternatively, one can measure residual dipolar couplings (RDCs) for natural abundance lipid samples to obtain segmental SCH order parameters. A theoretical mean-torque model provides acyl-packing profiles representing the cumulative chain extension along the normal to the aqueous interface. Equilibrium structural properties of fluid bilayers and various thermodynamic quantities can then be calculated, which describe the interactions with cholesterol, detergents, peptides, and integral membrane proteins and formation of lipid rafts. One can also obtain direct information for membrane-bound peptides or proteins by measuring RDCs using magic-angle spinning (MAS) in combination with dipolar recoupling methods. Solid-state NMR methods have been extensively applied to characterize model membranes and membrane-bound peptides and proteins, giving unique information on their conformations, orientations, and interactions in the natural liquid-crystalline state.
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