Biophysical Journal 90 6

Cover picture: Distinction of purple membrane (PM) extracellular (EC) and cytoplasmic (CP) sides using alternate contact mode atomic force microscopy in liquid (A-E). Topographic image (A). The phase image (B) reveals a contrast between the EC (darker) and CP (lighter) side of PM, not visible in panel A. A profile of B quantifies the phase shift C. High-resolution topographic images of the CP (D) and EC (E) side obtained by zooming, respectively, on the lighter and darker patches in B. Protrusions on the CP patches and a “smooth belt” around the EC patch are clearly visible in A and B (dashed line). Removing salt induces side-specific effects on PM observable by AC-AFM in air (F-J). The membrane has been rinsed with ultrapure water and dried at room temperature. The EC (pitted) and CP (cracked) sides of PM after drying (F). Higher magnifications of the EC patch (G and H). Hexagonal depressions related to the CP leaflet and pits are visible. Higher magnification of the CP cracked patch (I and J). Arrows point to larger holes crossing EC patches in F and G, and to trimers in J. The scale bar is 1 μm for A and B, 10° for C, 3 nm for D and E, 1 μm for F, 100 nm for G, 50 nm for H, 100 nm for I, and 50 nm for J. Panels F-I are topographic images and J is a phase image. See the article by Voïtchovsky et al. on page 2075.