²H-NMR in liquid crystals and membranes

Deuterium NMR spectroscopy is widely applicable to studies of the structure and dynamics of molecular solids, liquid crystals, and thin films of membrane lipids. The properties of soft nanomaterials are also accessible on the mesoscopic length scale intermediate between the molecular and bulk dimensions. For membrane lipids in the liquid-crystalline state, rapid axial averaging occurs about the director axis (the membrane normal). One can then relate the profiles of the order parameters |(CD)| of the individual C-²H labeled segments to average bilayer properties. These include the mean area per molecule and projected acyl chain length, the area compressibility modulus, and the radius of curvature for reverse hexagonal (H(II)) phase nanotubes. In addition, measurements of the relaxation rates for Zeeman order, R(1Z), and quadrupolar order, R(1Q), enable one to investigate the mean-squared amplitudes and time-scales of the fluctuations that underlie the thermodynamic properties. A unified interpretation is provided by a composite membrane deformation model, which fits simultaneously the frequency dependence and the angular anisotropy of the R(1Z) and R(1Q) relaxation rates. The results suggest the bilayer dynamics in the MHz regime can be modeled in terms of nematic-like deformations of the membrane hydrocarbon interior, together with axial rotations of the lipid acyl chains. A small contribution from internal segmental motions is found, which implies the bilayer microviscosity is comparable to that of a liquid hydrocarbon. Finally, the ²H-NMR relaxation rates of lipid bilayers containing cholesterol in the liquid-ordered phase suggest a dynamically more rigid bilayer, involving fast axial lipid rotations together with a reduction in collective bilayer deformations. Possible future applications include studies of liquid crystals and thin films of membrane lipids and surfactants, as well as lipid-protein systems.