Reversible unfolding of single RNA molecules by mechanical force

Here we use mechanical force to induce the unfolding and refolding of singl e RNA molecules: a simple RNA hairpin, a molecule containing a three-helix junction, and the P5abc domain of the Tetrahymena thermophila ribozyme. All three molecules (P5abc only in the absence of Mg2+) can be mechanically un folded at equilibrium, and when kept at constant force within a critical fo rce range. are bi-stable and hop between folded and-unfolded states. We det ermine the force-dependent equilibrium constants for folding/unfolding thes e single RNA molecules and the positions of their transition states along t he reaction coordinate.