Early Achievements—Purple Membrane Topographs

The plasma membrane of the archaebacteria Halobacterium sali-narium [H. salinarium) consists of 75% BR and 25% lipids (w/w). As a light-driven proton pump containing seven transmembrane a -helices, BR absorbs light (T- ax = 560 nm) and builds up an electrochemical potential across the inner cell membrane of H. salinarium. BR forms a 2D trigonal crystal in purple membranes (Fig. 17.2A). 

The topography of native purple membranes has been extensively studied at nanometer resolution using AFM in contact mode and oscillating mode in aqueous conditions (see 

The capability of AFM to measure forces in the range of tens of picoNewtons has stimulated its application to study intermolecular and intramolecular forces stabilizing the fold of membrane proteins. The protein of interest is pulled out of the biological membrane by the AFM probe, and the force-distance spectrum depicts the force required to unfold the molecule in distinct steps as well as the contour length of the unfolded pol q)eptide. BR is one of the bestcharacterized membrane proteins by single-molecule force spectroscopy (SMFS). - When force spectroscopy measurements were applied to polytopic membrane proteins, the technique was shown to be sufficiently sensitive to detect forces between the transmembrane helices. -  

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