Introduction to Atomic Force Microscopy (AFM) in Biology
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- Abstract
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Abstract
The atomic force microscope (AFM) has the unique capability of imaging biological samples with molecular resolution in buffer solution. In addition to providing topographical images of surfaces with nanometer? to angstrom?scale resolution, forces between single molecules and mechanical properties of biological samples can be investigated from the nanoscale to the microscale. Importantly, the measurements are made in buffer solutions, allowing biological samples to ?stay alive? within a physiological?like environment while temporal changes in structure are measured?e.g., before and after addition of chemical reagents. These qualities distinguish AFM from conventional imaging techniques of comparable resolution, e.g., electron microscopy (EM). This unit provides an introduction to AFM on biological systems and describes specific examples of AFM on proteins, cells, and tissues. The physical principles of the technique and methodological aspects of its practical use and applications are also described. Curr. Protoc. Protein Sci. 58:17.7.1?17.7.19. © 2009 by John Wiley & Sons, Inc.
Keywords: topography; force spectroscopy; manipulation; fluorescence microscopy
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PDF or HTML at Wiley Online LibraryTable of Contents
- Introduction
- Experimental Setup
- Applications
- Image Interpretation: Troubleshooting and Instrumental Effects
- Summary
- Acknowledgements
- Literature Cited
- Figures
- Tables
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Figure 17.7.1 Scheme showing how AFM produces images of surfaces. (A ) The sample surface is scanned with a fine tip. The interaction force between the tip and the surface causes the cantilever to bend. (B ) The sample is mounted in the AFM under a transparent cantilever holder (fluid cell). A laser beam is focused on the end of the cantilever. The deflection of the laser spot, induced by the bending of the cantilever, is measured by a photodiode detector and a feedback loop corrects the vertical distance by adjusting the piezo position. (C ) The AFM thereby produces a topographical image of the sample (surface of “constant force”). This figure was obtained from the M.E.Mueller‐Institute for Structural Biology, Basel, Switzerland, publication “Imaging, measuring and manipulating native biomolecular systems with the atomic force microscope” prepared by Daniel J. Müller, Ueli Aebi, and Andreas Engel. View Image -
Figure 17.7.2 Force versus distance curve. The deflection ( y ‐axis) of the free end of the cantilever is measured as the fixed end of the cantilever is brought vertically towards and then away from the sample surface. (1 ) The tip is initially not touching the surface—no cantilever deflection occurs. (2 ) As the tip approaches the surface, attractive forces cause it to “jump” into contact with the sample (note: this adhesive force is less pronounced when acquiring images under physiological conditions, i.e., in liquid, as compared to air). (3 ) Cantilever deflection becomes maximal when the tip is in contact with the sample (relates linearly to further approach to the sample as the tip is already in contact). If the tip indents the surface, elasticity forces related to the sample can be measured from the force curve. (4 ) As the tip retracts from the sample, adhesion or specific receptor‐ligand binding forces can cause the cantilever to stay attached to the sample some distance past the original point of contact. The point on the force curve where the tip eventually jumps off the surface can thus provide information about bond rupturing forces. Adapted from Digital Instruments. View Image -
Figure 17.7.3 (A ) Substrate with five cantilevers attached. (B ) Integration of a fine tip on the end of a cantilever. Images are from Veeco Probes: https://www.veecoprobes.com. View Image -
Figure 17.7.4 Adsorption density of purple membrane to mica as a function of the electrolyte concentration. Two micrograms of protein was incubated for 5 min at room temperature on 6‐mm‐diameter supports in buffer containing different amounts of (A ) monovalent or (B ) divalent ions. Adapted from Müller et al. (). View Image -
Figure 17.7.5 Force‐distance curves recorded on the extracellular surface of purple membrane. Force‐distance curves were recorded during the approach of sample and AFM tip under different electrolyte concentrations at constant pH (7.6). The dotted lines represent force‐distance curves recorded on purple membrane without electrostatic repulsion. Conditions: scan frequency: 1.97 Hz, scan range: 50 nm (512 pixels). Arrows (1 ) mark the onset of measurable electrostatic repulsion, whereas arrows (2 ) indicate the point of contact between tip and sample. Adapted from Müller et al. (). View Image -
Figure 17.7.6 High‐resolution topograph of a two‐dimensional crystal of the membrane protein aquaporin Z (AqpZ) using contact‐mode AFM recorded in buffer solution. Adapted from Scheuring et al. (). View Image -
Figure 17.7.7 Time‐lapse imaging: watching proteins at work. (A ) Movement of a single DNA molecule through an RNA polymerase complex (adapted from Kasas et al., ). (B ) Structural change induced in the nuclear basket of individual native nuclear pore complexes (NPCs) with addition of calcium ions. The same specimen area was imaged in two distinct conformational states. Three corresponding NPCs are marked by arrowheads. For a more quantitative comparison of the “closed” and “open” states, 30 corresponding NPCs were aligned and averaged, and their average radial height profiles computed. Scale bar, 100 nm (adapted from Stolz et al., ), (C ) Growth of an individual Aβ amyloid fibril adhering to a mica support (Goldsbury et al., ). View Image -
Figure 17.7.8 Image before (A ) and after (B ) unzipping of a full hexameric core of the Corynebacterium glutamicum S‐layer. (C ) Force‐distance curve corresponding to the change of the surface structure as shown in (A) and (B). The hexameric core is unzipped in a sequence of three dimer‐like rupture events (Scheuring et al., ,b). View Image -
Figure 17.7.9 (A ) Images after nanomanipulation of single desmin intermediate filaments. (B ) Typical lateral force versus displacement curve recorded during one nanomanipulation (adapted from Kreplak et al., ). View Image
Videos
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