Elastic Modulus of Suspended Membrane Protein Measured by Atomic Force Microscopy
Koji Sumitomo1, Ari M. Siitonen1, Chandra S. Ramanujan2, and Youichi Shinozaki1
1Materials Science Laboratory, 2University of Oxford
Using the atomic force microscopy (AFM), we have studied the mechanics of purple membrane (PM) suspended over nanostructures. By doing this we could evaluate the properties of the biological membrane free from interaction with the substrate. Biological membranes containing lipids and proteins are important components of bio-nanostructures with many potential applications, e.g. single-molecule biosensors, and reliable measurements of their mechanical properties are essential.
PMs of Halobacterium salinarium strain S9 were adsorbed on a Si substrate with nano-trenches fabricated using a photolithographic technique. After locating the suspended PM using the AFM in imaging mode, force-displacement measurements were performed. Figure shows AFM images of PM suspended over nano-trenches (a) before and (b) after indentation and (c) the force-displacement curve measured during indentation. The indentation mark left by the AFM tip can be clearly seen in (b) as indicated by the arrow. As the force is increased the suspended membrane was stretched and finally punctured. The force-displacement curve during membrane stretching reflects its mechanical property directly. To evaluate the surface tension, a simple theoretical model was made, and theoretical force curves were obtained to simulate the experimental force curves. From the best-fit curve, the elastic modulus of PM was estimated to be 8±1 MPa .
We have demonstrated that AFM measurements provide a very good approach to evaluate the mechanics of biological membranes in their native conditions. Proteins in the suspended potion of the membrane are not affected by the substrate and are more likely to function as they would in vivo. The next stage of this project is to measure changes in properties in response to an external stimulus such as mechanical stimulus, light irradiation, and chemical binding.
This research was supported in part by the Strategic International Cooperative Program, Japan Science and Technology Agency (JST).
 Ari M. Siitonen, et al., Appl. Surf. Sci., in press.
Fig.. AFM images of the PM suspended over nano-trenches: (a) before and (b) after indentation; (c) the force-displacement curve measured for suspended PM.
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