Changes in the surface structure of purple membrane upon illumination measured by atomic force microscopy

Bacteriorhodopsin (BR) patches with a diameter of 1 to 3 μm were investigated in their native state by atomic force microscopy (AFM) in buffer solution. The patches were immobilized deposited and investigated on mica in 150 mM KCl and 10 mM Tris-buffer at pH 8. Under this buffer condition they adsorb preferred with their extracellular side to the solid support mica. The structure of the two-dimensional light adapted crystals was resolved with an imaging force of about 100 pN up to a resolution of 13 Å. The topography of the surface gets smoother if an imaging force of 1000 pN was applied indicating that protruding structures are compressed. Upon illumination with white light, during imaging with a force of 200 pN, the surface structure of the BR lattice changed. The force- and light-induced structural changes were reversible.