Controlled Modulation of Lipid Bilayer State by a Photosensitive Membrane Effector

The lipid membrane matrix represents a 2-D liquid-crystal, the properties of which, at fixed other conditions, are locally modulated by the presence of effectors as e.g. cholesterol (passive) or proteins (passive and active). Not only does the incorporation of effectors into the host matrix locally or even globally modify the initial state of the host per se, most probably the state of the matrix in turn serves as a control tool for regulating the work of functional units (e.g. proteins). Results are presented on a membrane model system that was inoculated with a photosensitive and thus active variety of cholesterol. Azobenzene-cholesterol (azo-chol) exhibits a reversible trans-cis transition (365nm: trans- to cis-; 455nm: cis- to trans-). In a membrane, the azobenzene group, covalently connected to the cholesterol by an ester bond, is confined into the headgroup region. The system was explored by a combination of spectroscopic (UV-vis, NMR, mass spectroscopy), thermodynamic (Langmuir compression, calorimetry) and structural studies (X-ray/neutron reflectometry, grazing incidence X-ray diffraction). The conformational change of the guest upon illumination is coupled into the host system, inducing a transition of the whole membrane. The increased demand of headgroup area for the cis-azo-group pushes the membrane into a more compressed state, and vice versa for trans-azo-chol. The switching process between the two final states exhibits first-order kinetics. The state of the host bilayer is modulated as a response to the conformational switching of the guest effector via external light illumination. In a more general context, similar behavior may be found upon the conformational changes of membrane proteins during work.