Atomic force microscopy (AFM) allows the observation of surface structures of purple membrane (PM) in buffer solution with subnanometer resolution. This offers the possibility to classify the major conformations of the native bacteriorhodopsin (BR) surfaces and to map the variability of individual polypeptide loops connecting transmembrane α-helices of BR. The position, the variability and the flexibility of these loops depend on the packing arrangement of BR molecules in the lipid bilayer with significant differences observed between the trigonal and orthorhombic crystal forms. Cleavage of the Schiff base bond leads to a disassembly of the trigonal PM crystal, which is restored by regenerating the bleached PM. The combination of single molecule AFM imaging and single molecule force-spectroscopy provides an unique insight into the interactions between individual BR molecules and the PM, and between secondary structure elements within BR.
