NanoScale Designs May Be Aided By Highly Detailed 3D DNA Images

The general chemical structure of the DNA helix was described by James Watson and Francis Crick in 1953. Over the years that followed, scientists intensely studied the molecular structure of DNA to understand its behavior in vivo and to exploit its unique properties for nanotechnology purposes. Now, an international team of scientists working at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) has captured the first high-resolution 3D images from individual double-helix DNA segments attached at either end to gold nanoparticles. The images detail the flexible structure of the DNA segments, which appear as nanoscale jump ropes.

Using a cutting-edge electron microscopy (EM) technique, called individual-particle electron tomography (IPET), the researchers were able to visualize the shapes of the coiled DNA strands, which were sandwiched between polygon-shaped gold nanoparticles, and reconstruct high-resolution 3D images. The EM technique was coupled with a protein-staining process and sophisticated software that provided structural details to the scale of approximately 2 nanometers (two billionths of a meter).

We had no idea about what the double-strand DNA would look like between the nanogold particles,” noted senior study author Gang Ren, Ph.D., staff scientist in the Molecular Foundry at Berkeley Lab. “This is the first time for directly visualizing an individual double-strand DNA segment in 3-D.” The findings from this study were published recently in Nature Communications in an article entitled “Three-Dimensional Structural Dynamics and Fluctuations of DNA-Nanogold Conjugates by Individual-Particle Electron Tomography.”

Dr. Ren and his colleagues hope their unique imaging technique will aid in the use of DNA segments as building blocks for molecular devices that function as nanoscale drug-delivery systems, markers for biological research, and components for computer memory and electronic devices. Additionally, the research team speculates that the new method could also lead to images of important disease-relevant proteins that have proven elusive for other imaging techniques and of the assembly process that forms DNA from separate, individual strands.

The Berkeley Lab scientists flash froze samples to preserve their structure for study with cryo-EM imaging. The distance between the two gold particles in individual samples varied from 20 to 30 nanometers based on different shapes observed in the DNA segments. They then collected a series of tilted images of the stained objects and reconstructed 14 electron-density maps that detailed the structure of individual samples using the IPET technique. They gathered a dozen confirmations for the samples and found the DNA shape variations were consistent with those measured in the flash-frozen cryo-EM samples.

While the 3D reconstructions show the basic nanoscale structure of the samples, the investigators are looking at the next steps, which will be to work on improving the resolution to the subnanometer scale. “Even in this current state we begin to see 3D structures at 1- to 2-nanometer resolution,” Dr. Ren explained. “Through better instrumentation and improved computational algorithms, it would be promising to push the resolution to that visualizing a single DNA helix within an individual protein.”

In future studies, Dr. Ren noted that researchers could attempt to improve the imaging resolution for complex structures that incorporate more DNA segments as a sort of “DNA origami”—with the hope of building and better characterizing nanoscale molecular devices using DNA segments that can, for example, store and deliver drugs to targeted areas of the body. “DNA is easy to program, synthesize, and replicate, so it can be used as a special material to quickly self-assemble into nanostructures and to guide the operation of molecular-scale devices,” Dr. Ren stated. “Our current study is just a proof of concept for imaging these kinds of molecular devices’ structures.