Phospholipid Bilayer Membranes: Deuterium and Carbon-13 NMR Spectroscopy

Abstract

Solid-state 13C and 2H NMR measurements allow characterization of lipid membrane structure and dynamics and offer unique insights into their biophysical properties. Structural and dynamical formalisms provide a model-free reduction of 2H and 13C NMR lineshape and relaxation measurements in terms of mean-square amplitudes (order parameters) and reduced spectral densities (correlation times). Segmental order parameters are obtained from residual quadrupolar couplings (RQCs) and residual dipolar couplings (RDCs) that provide direct information about bilayer structural properties. Application of a mean-torque model gives the area per lipid and volumetric hydrocarbon thickness, and is important for validating molecular dynamics (MD) simulations. Moreover, NMR relaxation rates yield complementary information about multiscale bilayer dynamics if treated in a unified manner. Combination of frequency-dependent spin–lattice relaxation rates and anisotropic spin couplings allow unified interpretations of complementary 2H and 13C NMR experiments. Relaxometric measurements show that lipid mobility and collective order fluctuations are implicitly governed by the viscoelastic nature of the liquid-crystalline ensemble. Notably, the bilayer interior has a microviscosity similar to liquid hydrocarbons. Collective bilayer excitations emerge over mesoscopic length scales between the molecular and bilayer dimensions and are important for lipid organization and protein interactions. Keywords: magnetic dipolar coupling; molecular dynamics; nuclear spin–lattice relaxation; order parameter; phospholipid bilayer; quadrupolar coupling; solid-state NMR