Some new developments in solid-state nuclear magnetic resonance spectroscopic studies of lipids and biological membranes, including the effects of cholesterol in model and natural systems

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Abstract
High-field proton magic-angle sample-spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy is shown to yield high-resolution 1H spectra of smectic, nematic and hexagonal-II phase lipids, from which isotropic chemical shifts, order parameters and relaxation times (T1, T1ρ and T2) can be determined. Such experiments are possible because of the special form of the dipolar Hamiltonian in such systems. Resolution is about the same as that obtained with sonicated systems, using conventional NMR techniques. We also show that 13C MAS NMR spectra, of both fluid and solid phases, are even better resolved, and in some cases resonances can be observed in MAS NMR spectra which are not observable in sonicated systems. For example, essentially all of the carbon atoms in cholesterol (CHOL) can be readily detected and assigned in a lecithin–CHOL bilayer, using MAS, while few can be seen in sonicated bilayers. This leads directly to the observation of cholesterol in intact biological membranes, such as human myelin, where over 50 peaks can be observed, and ca. 40 of these resonances can be assigned to specific, single-carbon-atom sites in the membrane. In addition, a number of experiments with massively deuterated lipids are reported. Combination of cross-polarization techniques with MAS, and difference spectroscopy, leads to the observation of essentially pure sterol spectra (in the presence of lipid) and pure lipid spectra (in the presence of CHOL). Analysis of chemical-shift results indicates a substantial deshielding of chain carbon atom resonances caused by the presence of CHOL, due presumably to increased trans chain segments, an effect mirrored in variable temperature spectra of human myelin, and in goldfish myelin. Taken together, these results suggest a resurgence in NMR studies of membranes may soon occur.