NMR study of collective motions and bending rigidity in multilamellar system of lipid and surfactant bilayers

The frequency dependence of the longitudinal proton spin relaxation time T-1 was measured by field-cycling and standard NMR techniques at different temperatures in the liquid crystalline lamellar phases of bilayer systems, composed of lipids, nonionic surfactants, and lipid-surfactant mixtures. We show by our data analysis, comparing various motional models such as layer undulations (LUs) and relaxation by translational diffusion mediated reorientations (TR), that collective layer undulations with their typical T-1 similar to v behaviour determine the low frequency T-1 dispersion in both unoriented and glass plate-oriented bilayer systems. The angular dependence of the T-1 dispersion for the oriented bilayer system supports these findings and provides a more critical analysis of the two dimensional self-diffusion than in unoriented samples. The evaluated fitting parameters of the LU model allows, together with the measured second moment of the proton NMR signal for the lipid, calculation of the bending rigidity x(c) for these bilayers at different levels of hydration. The obtained values of;c, turn out to be too large compared with the literature. However, using recent LU models (B. Halle) which include the obvious couplings between neighbouring bilayers at low Larmor frequencies, the corrected x(c) of the fully hydrated membrane systems are comparable to those obtained from the standard videooptical experiments. Therefore proton spin relaxation measurements at low Larmor frequencies with the field-cycling technique are a suitable means to determine the bending rigidity x(c) of model membrane systems at low hydrations and of systems containing surfactants.