Artist’s conception of JILA’s advance in AFM design.
This Research in Action article was provided to LiveScience in partnership with the National Science Foundation.
To measure single molecules in a liquid, atomic force microscopes rely on tiny springboard devices known as cantilever probes. Molecules such as DNA attach to these probes and scientists measure the force exerted when the molecule pulls on the probe. Until recently, the cantilevers were coated with gold to improve their ability to reflect light.
However, research at JILA, a joint institute between the National Institute of Standards and Technology and the University of Colorado Boulder (JILA/NIST) suggests that stripping the cantilevers of their gold coating actually improves AFM precision and stability.
While building an ultra-stable AFM, JILA/NIST physicist Thomas Perkins and his team removed the cantilever's coating in an effort to eliminate any form of "noise" that might affect stability.
"We never would have looked at the gold coating if we hadn't built an ultra-stable AFM," Perkins says. Gold is a metal so we don't usually think of it as experiencing any kind of movement. But recent research has shown that gold itself is viscoelastic, i.e., having viscous and elastic properties. It can drift and creep. In addition, when gold-coated probes contact a liquid, the coating can degrade, a process often colloquially termed "cracking." The combination of movement and change in mechanical properties decreases the microscope's stability. When measuring forces at the piconewton scale — trillionths of a newton, which is a measure of force — the slightest variations can impact precision. One newton is roughly the weight of a small apple.
By removing the cantilever's coating, the team improved the AFM's stability at room temperature more than 10-fold. Equally good news — they can apply their approach to commercially available cantilevers with a 60-second chemical bath to improve the stability of existing commercial AFMs.
With the cantilever enhancement, AFM can now compete with optical tweezers to pull on proteins with a delicate touch. Reduced drift will also let researchers improve high-resolution imaging of membrane proteins in their native lipid bilayer state. Preserving the proteins in their native environment offers an advantage over other imaging methods such as nuclear magnetic resonance and x-ray crystallography. These latter approaches use detergents to extract the proteins from the lipid bilayer prior to imaging. This process makes it more difficult to characterize the proteins.
A clearer picture of how membrane proteins fold and unfold will improve models depicting drug-protein interactions. This data is critical for pharmaceutical research, since 50 percent of current and future drugs target membrane proteins.
Editor's Note: Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation. See the Research in Action archive.