Observing and Controlling the Folding Pathway of DNA Origami at the Nanoscale

DNA origami is a powerful method to fold DNA into rationally designed nanostructures that holds great promise for bionanotechnology. However, the folding mechanism has yet to be fully resolved, principally due to a lack of data with single molecule resolution. To address this issue, we have investigated in detail, using atomic force microscopy, the morphological evolution of hundreds of individual rectangular origamis in solution as a function of temperature. Significant structural changes were observed between 65 and 55 degrees C both for folding and melting, and six structural intermediates were identified. Under standard conditions, folding was initiated at the edges of the rectangle and progressed toward the center. Melting occurred through the reverse pathway until the structures were significantly disrupted but ended through a different pathway involving out-of-equilibrium chainlike structures. Increasing the relative concentration of center to edge staples dramatically modified the folding pathway to a mechanism progressing from the center toward the edges. These results indicate that the folding pathway is determined by thermodynamics and suggest a way of controlling it.