Azobenzene-Cholesterol as a Photoactivator in Biomimetic Membranes: 2. Membrane Structure

Properties of biomembranes are modified in presence of additives such as cholesterol (chol) or proteins. Azobenzene-cholesterol (azo-chol) is a photoactive variety of chol since its azo-headgroup exhibits a reversible change of the conformation upon illumination (365nm: trans to cis; 455nm: cis to trans). Grazing incidence X-ray diffraction (GIXD) was applied to resolve the lateral structure within biomimetic monolayers consisting of a 1,2-dipalmitoyl-sn-phosphatidyl-choline (DPPC) matrix with embedded azo-chol in either of the two conformational states. A pure DPPC monolayer and also a DPPC monolayer with inactive chol were used as control systems. The results report on the symmetry of the 2D-lattice and the inter-chain distance dxy. The tilt angle of the lipid chains was calculated from the lattice parameters and the geometrical boundary conditions. The lattice parameters are modified when azo-chol in the host membrane is in all-trans state as compared to the control systems. With all-cis azo-chol, the structure is broken. The findings show the capacity of azo-chol for switching the lateral structure of the host lipid membrane though photoactivation.