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 angstrom-scale amplitude undulations of freely suspended planar lipid bilayers, so-called black lipid membranes (BLM’s), over a previously not accessible spread of frequencies (relaxation times ranging from ${10}^{-2}$ to ${10}^{-6}\mathrm{s})$ and wave vectors $(250\mathrm{}{\mathrm{cm}}^{\mathrm{-}1}<q<\mathrm{}35\mathrm{}000{\mathrm{cm}}^{\mathrm{-}1}).\mathrm{}$ 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 agreement with the theoretical predictions. In particular, the transition of the transverse 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-vector regime covered are dominated by the membrane tension, while membrane curvature does not contribute. The binding of the protein streptavidin to the BLM via membrane anchored specific binders (receptors) causes a drastic change 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 probably caused by a steric hindrance of the transverse shear motions of the lipid by the tightly bound proteins.