Advancing Wireframe DNA Nanostructures Using Single-Molecule Fluorescence Microscopy Techniques.

DNA nanotechnology relies on the molecular recognition properties of DNA to produce complex architectures through self-assembly. The resulting DNA nanostructures allow scientists to organize functional materials at the nanoscale and have therefore found applications in many domains of materials science over the past several years. These scaffolds have been used to position proteins, nanoparticles, carbon nanotubes, and other nanomaterials with high spatial resolution. In addition to their remarkable performance as frameworks for other species, DNA constructs also possess interesting dynamic properties, which have led to their use in logic circuits, drug delivery vehicles, and molecular walkers. Although DNA nanostructures have become increasingly complex, the development of tools to study them has lagged. Currently, gel electrophoresis, dynamic light scattering, and ensemble fluorescence measurements are widely used to characterize DNA-based assemblies. Unfortunately, ensemble averaging in these methods obscures malformed structures and may mask properties associated with structure, length, and shape in polydisperse samples. While atomic force microscopy allows for the determination of morphology at the single-molecule level, this technique cannot typically be used to assess the dynamic properties of these constructs. To analyze the function of DNA-based devices such as molecular motors and reconfigurable nanostructures in real time, new single-molecule techniques are required. This Account details the work from our laboratories toward developing single-molecule fluorescence (SMF) methodologies for the structural and dynamic characterization of wireframe DNA nanostructures, one at a time. The methods described herein provide us with two separate yet related sets of information: First, we can statically examine the nanostructures one by one to assess their robustness, structural fidelity, and morphology. This is primarily done using two-color stepwise photobleaching, wherein we can examine the subunit stoichiometry of our assemblies before and after various perturbations to the structures. For example, we can introduce length mismatches to cause the nanotube to bend or perform strand displacement reactions to generate single-stranded, flexible analogues of our materials. Second, due to the unmatched spatiotemporal resolution of SMF techniques, we can study the dynamic character of these assemblies by implementing structural changes to the nanotube and monitoring them in real time. With this structural and dynamic information in hand, our groups have additionally developed new tools for the improved construction of DNA nanotubes, inspired by solid-phase DNA synthesis. By assembling the nanotubes in a stepwise manner, highly monodisperse nanostructures of any desired length can be made without a template strand. In this way, unique building blocks can also be added sequence-specifically, allowing for the production of user-defined scaffolds to organize nanoscale materials in three dimensions. This method, in combination with our imaging and analysis protocols, may be extended to assemble and inspect other supramolecular constructs in a controlled manner. Overall, by combining synthesis, characterization, and analysis, these single-molecule techniques hold the potential to advance the study of DNA nanostructures and dynamic DNA-based devices.