Measuring changes in membrane thickness by scanning tunneling microscopy.
We investigated the feasibility of using the scanning tunneling microscope (STM) as a morphometric tool to measure the thickness of biomembranes. Planar monolayers of oriented purple membrane (PM) were prepared, nitrogen-dried or freeze-etched, and coated with metal. PM thickness was quantified by STM and transmission electron microscopy. STM calibration and the effect of contamination-mediated surface deformation on measurements of PM thickness were evaluated. The thickness of PM attached to mica and glass and the effect of papain on PM thickness were also examined. The apparent thickness of enzymatically modified PM increased after papain treatment. The mean thickness of both nitrogen-dried PM on mica and freeze-etched PM on glass was 4.6 nm. After papain treatment PM thickness on mica increased to 4.8 nm and on glass to 5.4 nm. These results demonstrate that STM analysis of metal-coated planar membrane monolayers can be used to measure changes in average membrane thickness at sub-nanometer resolution.