Free energy landscape of salt-actuated reconfigurable DNA nanodevices.

Achieving rapid, noninvasive actuation of DNA structures is critical to expanding the functionality of DNA nanotechnology. A promising actuation approach involves introducing multiple, short pairs of single-stranded DNA overhangs to components of the structure and triggering hybridization or dissociation of the overhangs via changes in solution ionic conditions to drive structural transitions. Here, we reveal the underlying basis of this new approach by computing via molecular simulations the free energy landscape of DNA origami hinges actuated between open and closed states. Our results reveal how the overhangs collectively introduce a sharp free-energy minimum at the closed state and a broad energy barrier between open and closed states and how changes in ionic conditions modulate these features of the landscape to drive actuation towards the open or closed state. We demonstrate the critical role played by hinge confinement in stabilizing the hybridized state of the overhangs and magnifying the energy barrier to dissociation. By analyzing how the distribution of overhangs and their length and sequence modulate the energy landscape, we obtain design rules for tuning the actuation behavior. The molecular insights obtained here should be applicable to a broad range of systems involving DNA hybridization within confined systems.