Surfactant-induced alterations of lecithin molecular dynamics in bilayers studied by quasielastic neutron scattering and solid-state MMR

Quasielastic neutron scattering (QENS) measurements were employed to study changes in high-frequency dynamics of dipalmitoyl-phosphatidyl-choline (DPP C) bilayers induced by small amounts of nonionic surfactants (tetra-ethylen eglycol-mono-n-dodecyl ether, C12E4) The experiments were performed at thre e energy resolutions probing different frequency domains (GHz to lower THz range) of molecular motion and at two temperatures, corresponding to the cr ystal-like gel phase (T = 20 degrees C) and the fluid phase (T = 50 degrees C) of the bilayer. Two orientations of the bilayer stack were studied to o btain 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 flui d DPPC bilayer, we observed drastic changes of lipid dynamics in the freque ncy domain which is dominated by diffusive motions of the whole molecule. T he presence of surfactant increased the Lipid in-plane diffusion constant b y 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%. Solid-state deuterium NMR of fluid DPPC bilayers showed that the surfactant caused a reduction of the or der parameter of the lipid chains and changed the shape of the order parame ter profile. In the high-frequency domain where kink motions of the lipid c hains dominate the dynamics, no surfactant effects were observed. In a time averaged picture, the results suggest a surfactant-induced spread of the l ipid chains in the bilayer plane and a concomitant reduction of bilayer thi ckness. For gel phase bilayers, no surfactant- induced alterations of lipid dynamics were detected.