Collective membrane motions in the mesoscopic range and their modulation by the binding of a monomolecular protein layer of streptavidin studied by dynamic light scattering

Using a dedicated dynamic light scattering setup, we have studied the angst rom-scale amplitude undulations of freely suspended planar lipid bilayers, so-called black lipid membranes (BLM's), over a previously not accessible s pread of frequencies (relaxation times ranging from 10(-2) to 10(-6) s) and wave vectors (250 cm(-1)<q<35 000 cm(-1)). This allowed a critical test of a simple hydrodynamic theory of collective membrane modes, and the results obtained for a synthetic lecithin BLM are found to be in excellent agreeme nt with the theoretical predictions. In particular, the transition of the t ransverse shear mode of a BLM between an oscillatory or propagating regime and an overdamped regime by passing through a bifurcation point was clearly observed. It is shown that the collective motions in the time- and wave-ve ctor regime covered are dominated by the membrane tension, while membrane c urvature does not contribute. The binding of the protein streptavidin to th e BLM via membrane anchored specific binders (receptors) causes a drastic c hange in frequency and amplitude of the collective motions, resulting in a drastic increase of the membrane tension by a factor of 3. This effect is p robably caused by a steric hindrance of the transverse shear motions of the lipid by the tightly bound proteins. [S1063-651X(99)13705-X].