Collective Lipid Bilayer Dynamics Excited by Surface Acoustic Waves

We use standing surface acoustic waves to induce coherent phonons in model lipid multilayers deposited on a piezoelectric surface. Probing the structure by phase-controlled stroboscopic x-ray pulses we find that the internal lipid bilayer electron density profile oscillates in response to the externally driven motion of the lipid film. The structural response to the well-controlled motion is a strong indication that bilayer structure and membrane fluctuations are intrinsically coupled, even though these structural changes are averaged out in equilibrium and time integrating measurements. Here the effects are revealed by a timing scheme with temporal resolution on the picosecond scale in combination with the sub-nm spatial resolution, enabled by high brilliance synchrotron x-ray reflectivity.

Figure 1

(a) Multilamellar stacks of lipid bilayers deposited on the piezoelectric ${\mathrm{LiNbO}}_3$ substrate of a SAW device. (b) By application of a rf signal to the IDTs, standing waves or propagating SAW pulses are induced. The ultrafast response of (c) the averaged bilayer electron density profile $\mathrm{\Delta }\rho (z)$ is measured by (d) time resolved (phase locked) x-ray reflectivity experiments.

Figure 2

The integrated intensities $I(h,\varphi )$ of the first four diffraction orders $h$ of the multilamellar lipid stack plotted as a function of relative phase $\varphi$. Individual intensity traces $I(h,\varphi )$ are out of phase, pointing to a complex interaction between the SAW and the lipid multilayer structure. The plots have been shifted for clarity.

Figure 3

(a) Structural dynamics of the lipid bilayer in response to the acoustic excitation by the SAW, as quantified by the time resolved electron density profile, shown for selected phase angles $\varphi$ along with the equilibrium reference profile (dashed line). Pronounced variations of the resulting relative electron density $\mathrm{\Delta }\rho (z,\varphi )$ are observed. The density variations are accompanied by a variation of membrane thickness. (b) Characteristic bilayer parameters (membrane thickness and headgroup width) extracted from $\mathrm{\Delta }\rho (z,\varphi )$. The maxima in the headgroup width $d_{hw}$ are accompanied by minima of the membrane thickness $d_{hh}$, and (c) a reduction of the electron density in the headgroup region (negative excess density) compensated for by a density increase in the tail region (positive excess density).