Atomic force microscopy of native purple membrane

Atomic force microscopy (AFM) allows the observation of surface structures of purple membrane (PM) in buffer solution with subnanometer resolution. Th is offers the possibility to classify the major conformations of the native bacteriorhodopsin (BR) surfaces and to map the variability of individual p olypeptide loops connecting transmembrane alpha-helices of BR. The position , the variability and the flexibility of these loops depend on the packings arrangement nf RR molecules in the lipid bilayer with significant differen ces 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 singl e molecule AFM imaging and single molecule force-spectroscopy provides an u nique insight into the interactions between individual BR molecules and the PM, and between secondary structure elements within BR. (C) 2000 Elsevier Science B.V. All rights reserved.