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