The geometry of domains in phospholipid bilayers of binary (1:1) mixtures of synthetic lecithins with a difference in chain length of four methylene groups has been studied by two independent, direct and complementary methods. Grazing incidence diffraction of neutrons provided gel domain sizes of less than 10 nm in both the gel and the coexistence phase of the mixture, while no domains were detected for the fluid phase. For the coexistence region, the neutron data suggest that domains grow in number rather than in size with decreasing temperature. Atomic force microscopy was used to study gel phase size and shape of the domains. The domains imaged by atomic force microscopy exhibit a rather irregular shape with an average size of 10 nm, thus confirming the neutron results for this phase. The good agreement between atomic force microscopy and neutron results, despite the completely different nature of their observables, has potential for the future development of refined models for the interpretation of neutron data from heterogeneous membranes in terms of regularly spaced and spatially extended scatterers.

Quasielastic neutron scattering (QENS) at two energy resolutions (1 and 14 microeV) was employed to study high-frequency cholesterol motion in the liquid ordered phase (lo-phase) of oriented multilayers of dipalmitoylphosphatidylcholine at three temperatures: T = 20 degrees C, T = 36 degrees C, and T = 50 degrees C. We studied two orientations of the bilayer stack with respect to the incident neutron beam. This and the two energy resolutions for each orientation allowed us to determine the cholesterol dynamics parallel to the normal of the membrane stack and in the plane of the membrane separately at two different time scales in the GHz range. We find a surprisingly high, model-independent motional anisotropy of cholesterol within the bilayer. The data analysis using explicit models of molecular motion suggests a superposition of two motions of cholesterol: an out-of-plane diffusion of the molecule parallel to the bilayer normal combined with a locally confined motion within the bilayer plane. The rather high amplitude of the out-of-plane diffusion observed at higher temperatures (T >/= 36 degrees C) strongly suggests that cholesterol can move between the opposite leaflets of the bilayer while it remains predominantly confined within its host monolayer at lower temperatures (T = 20 degrees C). The locally confined in-plane cholesterol motion is dominated by discrete, large-angle rotational jumps of the steroid body rather than a quasicontinous rotational diffusion by small angle jumps. We observe a significant increase of the rotational jump rate between T = 20 degrees C and T = 36 degrees C, whereas a further temperature increase to T = 50 degrees C leaves this rate essentially unchanged.

Quasielastic neutron scattering (QENS) measurements were employed to study changes in high-frequency dynamics of dipalmitoyl-phosphatidyl-choline (DPPC) bilayers induced by small amounts of nonionic surfactants (tetra-ethyleneglycol-mono-n-dodecyl ether, C 12 E 4 ). The experiments were performed at three energy resolutions probing different frequency domains (GHz to lower THz range) of molecular motion and at two temperatures, corresponding to the crystal-like gel phase (T ) 20 °C) and the fluid phase (T ) 50 °C) of the bilayer. Two orientations of the bilayer stack were studied to obtain information about the anisotropy of the dynamics with respect to the in-plane and the out-of-plane lipid motion. At 5 mol % surfactant in a fluid DPPC bilayer, we observed drastic changes of lipid dynamics in the frequency domain which is dominated by diffusive motions of the whole molecule. The presence of surfactant increased the lipid in-plane diffusion constant by 50% and the spatial extension of this motion by 25%. In contrast, the out-of-plane lipid motion showed a reduction of the diffusion constant by 60% and its spatial extension was reduced by 40%. Solidstate deuterium NMR of fluid DPPC bilayers showed that the surfactant caused a reduction of the order parameter of the lipid chains and changed the shape of the order parameter profile. In the high-frequency domain where kink motions of the lipid chains dominate the dynamics, no surfactant effects were observed. In a time averaged picture, the results suggest a surfactant-induced spread of the lipid chains in the bilayer plane and a concomitant reduction of bilayer thickness. For gel phase bilayers, no surfactant-induced alterations of lipid dynamics were detected.

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