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Understanding biomolecular interaction is one of the most important questions of biological physics. In fact, a protein's function is often closely related to its three-dimensional structure and the pathway it follows to fold into that structure. So much so that there are a large number of diseases that are associated with protein misfolding, such as Alzheimer's, Parkinson's, and Huntington's diseases. Studying the energetics of the protein folding process can give insight into that protein's function, as well as the processes that may lead to protein misfolding and disease.

The atomic force microscope (AFM) now allows one to unfold proteins or stretch molecules essentially by hand. In these experiments, proteins in solution are deposited on a gold surface and the AFM's microscopic cantilever arm is repeatedly dipped into the protein solution. During this cycle, the tip of AFM cantilever attaches to individual molecules, and the attached molecule can subsequently be stretched by pulling the cantilever away from the sample surface. The protein's reaction force is measured by monitoring the bending of the cantilever throughout the manipulation of the molecule.

Kiang's group used the atomic force microscope to pull a single domain of the heart muscle protein titin, measuring the force required to stretch and unfold that protein. With help from modern statistical theory, i.e. the Jarzynski's Equality, the group mapped the protein's free energy landscape along the entire unfolding trajectory, which had not been accomplished to date by any other methods. The new technique can be applied to most any biomolecule, complex, or molecular interaction.

Harris, N. C., Song, Y., and Kiang, C.-H.Experimental Free Energy Surface Reconstruction from Single-Molecule Force Spectroscopy using Jarzynski's Equality . PRL 2007, 99: 068101. (Reprint)

Press Releases
(APS March 2007 newsletter Mapping Protein Folding)
(Science News Pulling Strings: Stretching proteins can reveal how they fold)
(Rice News & Media Relations press release Protein pulling: Learning how proteins fold by pulling them apart)
(PhysicsWorld Microscope unravels the intricacies of protein folding)
The Best of Small Tech Researcher of the Year award (2007 Best of Small Tech Award Researcher of the Year)



Protein Stretching

Single molecule force spectroscopy uses a microscopic AFM cantilever to grab individual proteins attached to a gold surface. This protein consists of eight repeats of the I27 domain of the heart muscle protein titin, a domain found in the extensible region of the striated muscle. The protein is stretched by retracting the cantilever from the sample surface at a constant velocity, and the force required to manipulate the molecule is monitored by the bending of the cantilever.