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 d(w) almost-equal-to 10 angstrom) was studied by spin echo spectroscopy in the wave vector range 0.03 angstrom-1 < q < 0.125 angstrom-1, i.e. at qd(w) less-than-or-equal-to 1. The linewidth of the excitations exhibits a scaling law GAMMA(q) is-proportional-to 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 (omega is similar to q2)-dependence is predicted and its calculated frequencies are in accordance with the experimentally observed order of magnitude.