Mechanism of DNA origami folding elucidated by mesoscopic simulations
DNA nanotechnology leverages the canonical base-pairing rules and geometry of DNA to create highly precise nanoscale structures with many potential applications. While the design and fabrication of DNA nanostructures is well-established, the self-assembly process that produces these structures is still poorly understood, especially for DNA origami that involve the assembly of hundreds of strands. Many experimental and computational efforts have sought to better understand DNA origami folding, but the small length and time scales of individual binding events and the long timescale over which folding occurs have posed significant challenges. Here, we present a new mesoscopic model that uses a switchable force field to capture the mechanical behavior of single- and double-stranded DNA motifs and transition between them at a coarseness level of up to 8 nucleotides per particle, allowing access to the long assembly timescales of DNA origami up to several kilobases in size. Brownian dynamics simulations of 4-helix bundle (4HB) structures using this model reveal a hierarchical folding process involving the zipping of structural domains into a partially folded precursor structure followed by gradual crystallization into the final structure. We elucidate the role of hybridization strength, scaffold routing, and staple design in the folding order and kinetics. Simulation of larger 32HB structures reveals heterogeneous staple incorporation kinetics and frequent trapping in metastable states, as opposed to smaller, more accessible structures like the 4HB, which exhibit first-order kinetics and virtually defect-free folding. The development of this model opens an avenue to better understand and design DNA nanostructures for improved yield and folding performance.