A Lipid Photoswitch Controls Fluidity in Supported Bilayer Membranes

Abstract

Supported lipid bilayer (SLB) membranes are key elements to mimic membrane interfaces on a planar surface. Here, we demonstrate that azobenzene photolipids (<i>azo</i>-<b>PC</b>) form fluid, homogeneous SLBs. Diffusion properties of <i>azo</i>-<b>PC</b> within SLBs were probed by fluorescence microscopy and fluorescence recovery after photobleaching. At ambient conditions, we find that the <i>trans</i>-to-<i>cis</i> isomerization causes an increase of the diffusion constant by a factor of two. Simultaneous excitation with two wavelengths and variable intensities furthermore allows to adjust the diffusion constant <i>D</i> continuously. X-ray reflectometry and small-angle scattering measurements reveal that membrane photoisomerization results in a bilayer thickness reduction of ∼0.4 nm (or 10%). While thermally induced back-switching is not observed, we find that the <i>trans</i> bilayer fluidity is increasing with higher temperatures. This change in diffusion constant is accompanied by a red-shift in the absorption spectra. Based on these results, we suggest that the reduced diffusivity of <i>trans</i>-<i>azo</i>-<b>PC</b> is controlled by intermolecular interactions that also give rise to H-aggregate formation in bilayer membranes.