Salt-induced conformational switching of a flat rectangular DNA origami structure.

A rectangular DNA origami structure is one of the most studied and often used motif for applications in DNA nanotechnology. Here, we present two assays to study structural changes in DNA nanostructures and reveal a reversible rolling-up of the rectangular DNA origami structure induced by bivalent cations such as magnesium or calcium. First, we applied one-color and two-color superresolution DNA-PAINT with protruding strands along the long edges of the DNA origami rectangle. At increasing salt concentration, a single line instead of two lines is observed as a first indicator of rolling-up. Two-color measurements also revealed different conformations with parallel and angled edges. Second, we placed a gold nanoparticle and a dye molecule at different positions on the DNA origami structure. Distance dependent fluorescence quenching by the nanoparticle reports on dynamic transitions as well as it provides evidence that the rolling-up occurs preferentially along the diagonal of the DNA origami rectangle. The results will be helpful to test DNA structural models and the assays presented will be useful to study further structural transitions in DNA nanotechnology.