Measuring changes in membrane thickness by scanning tunneling microscopy

Force-induced conformational change of bacteriorhodopsin

The cytoplasmic surface topography of purple membranes imaged by the atomic force microscope depends mainly on the force applied to the stylus. Imaged at forces of 300 pN, individual bacteriorhodopsin molecules reveal two domains. The resulting donut-shaped trimers reversibly transform into structures exhibiting three prominent protrusions when scanned at 100 pN. In parallel, the height of the protein moiety above the lipid layer increases from 4 A to 6 A. From the known structure of bacteriorhodopsin it appears that this change may be related to a bending of the most prominent cytoplasmic loop.

Purple membrane multilayers: detailed structure and photoelectric characteristics of bacteriorhodopsin

The structure of the Langmuir purple membrane (PM) monolayer was studied by scanning tunneling microscopy (STM). The topography of both the external and cytoplasmic sides of the monolayer was investigated. The thickness of the PM monolayer was determined to be about 4 nm. PM films include specific oval 'crater'-like structures of 50-60 nm in diameter.

Scanning tunneling microscopy of planar biomembranes

We combined planar membrane monolayer techniques with scanning tunneling microscopy (STM) to measure the thickness of metal-coated purple membrane (PM) isolated from Halobacterium halobium. Although the metal coating precluded obtaining high-resolution lateral information, it facilitated obtaining high-resolution vertical information. For example, the apparent mean thickness of planar PM and variations in thickness of enzyme-treated PM could be detected and quantified at sub-nanometer resolution.