Neutron Spin Echo Study of Membrane Undulations in Lipid Multibilayers

Abstract

The undulatory excitations of highly ordered stacks of lipid bilayers at a high hydration level (19 weight % of water; water thickness dw ≈ 10 Å) was studied by spin echo spectroscopy in the wave vector range 0.03 Å-1 < q < 0.125 Å-1, i.e. at qdw ⩽ 1. The linewidth of the excitations exhibits a scaling law Γq ∝ q2.5±0.5. Undulations of a free membrane are expected to show a q3-behaviour and stacks of coupled bilayers a q2-dependence. Although both cases are still possible according to our scaling analysis, they can be ruled out as the frequencies expected from theory are a factor of 100 to 1000 higher than the observed ones. Our data seem to favour a recent theory by Seifert and Langer taking into account that the bilayer consists of two single layers coupled by friction. A new mode with a (ω ∼ q2)-dependence is predicted and its calculated frequencies are in accordance with the experimentally observed order of magnitude.