Neutron spin echo study of membrane undulations in lipid multibilayers

The undulatory excitations of highly ordered stacks of lipid bilayers at a high hydration level (19 weight % of water; water thickness dw « 10 Á) was studied by spin echospectroscopy in the wave vector range 0.03 Á-1 < q < 0.125 A-1, ie. at qdf, S 1. The linewidth ofthe excitations exhibits a scaling law rq oc <j2-5-0'5. Undulations of a free membrane are expectedto show a <j3-behaviour and stacks of coupled bilayers a <j2-dependence. Although both cases arestill possible according to our scaling analysis, they can be ruled out as the frequencies expectedfrom theory are a factor of 100 to 1000 higher than the observed ones. Our data seem to favour arecent theory by Seifert and Langer taking into account that the bilayer consists of two singlelayers coupled by friction. A new mode with a (a> - q2)-dependence is predicted and its calculatedfrequencies are in accordance with the experimentally observed order of magnitude.