NMR-STUDY OF COLLECTIVE MOTIONS AND BENDING RIGIDITY IN MULTILAMELLARSYSTEM OF LIPID AND SURFACTANT BILAYERS

The frequency dependence of the longitudinal proton spin relaxation ti me T-1 was measured by field-cycling and standard NMR techniques at di fferent temperatures in the liquid crystalline lamellar phases of bila yer systems, composed of lipids, nonionic surfactants, and lipid-surfa ctant mixtures. We show by our data analysis, comparing various motion al models such as layer undulations (LUs) and relaxation by translatio nal diffusion mediated reorientations (TR), that collective layer undu lations with their typical T-1 similar to v behaviour determine the lo w 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 c ritical analysis of the two dimensional self-diffusion than in unorien ted 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 neighbo uring bilayers at low Larmor frequencies, the corrected x(c) of the fu lly hydrated membrane systems are comparable to those obtained from th e standard videooptical experiments. Therefore proton spin relaxation measurements at low Larmor frequencies with the field-cycling techniqu e are a suitable means to determine the bending rigidity x(c) of model membrane systems at low hydrations and of systems containing surfacta nts.