Collective membrane motions of high and low amplitude, studied by dynamic light scattering and micro-interferometry

Undulations of lipid bilayers were experimentally studied for the two limit ing cases of high and weak lateral tension using two well established model systems: freely suspended planar lipid bilayers, so-called black lipid mem branes (BLM) for high-tension studies and large unilamellar vesicles (LUV) for measurements at weak tension. This variation in tension results in chan ges of undulation amplitudes from several hundred nm (LUV) down to 1 nm (BL M), thus requiring different physical methods for their detection. We have employed microinterferometric techniques (RICM) for studying the regime of weak tension and dynamic light scattering (DLS) for that of high tension. T he dedicated DLS set-up allowed the measurements of undulations over a wide wave vector range of 250 < q/cm(-1) < 35 000 cm(-1). This enabled the obse rvation of collective membrane modes in two regimes, the oscillating one at low q and the overdamped regime at high q. The transition between both reg imes at the bifurcation point is rather abrupt and depends on the lateral t ension of the bilayer, as is demonstrated by comparing the dispersion curve s of pure lipid and of lipid-cholestrol BLMs over the same q-range. The DLS measurements allowed a critical test of a hydrodynamic theory of the dispe rsion behaviour of membrane collective modes under tension. The DLS measure ments are compared with RICM results of undulatory excitations of giant ves icles weakly adhering to substrates in the 10(-6)-2.5 x 10(-7) m wavelength regime and at low frequencies (0.1-25 Hz). Experimental evidence for the s trong decrease in the relaxation rate by the hydrodynamic coupling of the m embrane with the wall is established.