Solid-state deuterium NMR spectroscopy of membranes

Solid-state deuterium ( ² H) NMR spectroscopy provides a unique tool for lipid membrane investigations. Knowledge of the average structure is obtained from solid-state ² H NMR lineshapes through principal values of the static or motionally averaged coupling tensors due to quadrupolar interactions. For randomly oriented multilamellar lipids or aligned membranes, this technique provides residual quadrupolar couplings (RQC) of the individual C- ² H labeled segments. The RQC values are used to calculate the segmental order parameters S _CD ⁽ⁱ⁾ for each segment position (i), which are related to the average membrane properties. The corresponding dynamical information is acquired from the tensor fluctuations, which depend on the mean-squared amplitudes and rates of the motions. Fluctuations of the coupling Hamiltonian due to the various membrane dynamics cause the relaxation. The ² H solid-state NMR relaxation methods facilitate studying the hierarchical dynamics of liquidcrystalline membranes over wide length and timescales. Model-free interpretation of spin-lattice relaxation rates as a function of segmental order parameters enables understanding the complex lipid dynamics. Notably, the squarelaw functional dependence of R _1Z rates and order parameters indicates the collective segmental dynamics, which in turn explain the liquid-crystalline material properties of lipid bilayer. Using solid-state ² H NMR relaxation, the influences of the acyl length, polyunsaturation, lipid polar head groups, cosurfactants, water, and incorporation of sterols are accessible in terms of the bilayer viscoelastic properties. These methods have been extensively applied to characterize model membranes and membrane-bound peptides as well as proteins for obtaining unique information on their conformations, orientation, and interactions.