optical or magnetic devices. But these measurements
are performed in ambient conditions, which imply
thermal fluctuations of the molecule, and are restricted to relatively long DNA branches.
But Pawlak and his team discovered a better way
to manipulate DNA using cyro-force spectroscopy.
The scientists placed only a few nanometer-long
DNA strands containing 20-cytosine nucleotides
on a gold surface. At a temperature of 5 Kelvin,
one end of the DNA strand was then pulled
upwards using the tip of an atomic force microscope. In the process, the individual components
of the strand freed themselves from the surface little
by little. This enabled the physicists to record their
elasticity as well as the forces required to detach the
DNA molecules from the gold surface.
“Using cryo-force spectroscopy combined with molec-
ular dynamic simulations, we provided complementary
results with the detection of the mechanics of a DNA
branch [significantly smaller than previously possible],”
Pawlak told Laboratory Equipment. “We essentially
mechanically unpeeled the DNA, and stretched it like
a rubber band. The longer the detached piece of DNA,
According to Pawlak’s paper published in Nature
Communications, this is because the individual compo-
nents of the DNA behave like a chain of multiple coil
springs connected to one another.
Resultant computer simulations verified that DNA is
detached discontinuously from the surface. This is due
to the breaking up of bonds between the cytosine bases
and the DNA backbone from the gold surface, and their
abrupt movements over the gold surface. The theoretical
elasticity values correlate very closely with the experiments and confirm the model of serially arranged springs.
All in all, Pawlak says his expectations for a proof of
concept experiment were blown out of the water.
“The scientific outputs went beyond our expecta-
tions,” Pawlak said. “Atomic-scale AFM images revealed
the fine structural details of the molecule adsorbed on
gold and their tendency to form self-assemblies, such as
in DNA-origami. Overall, our investigations provide new
insights into the structural and mechanical properties of a
single strand on a surface without thermal motions.”
Cryo-spectroscopy turned out to be the shining star
in Pawlak’s experiments, especially when manipulating
short DNA strands. In the past, cryo-electron micros-
copy, or cryo-EM, was used for DNA research. This
technique, which experienced a surge in popularity in
the last couple of years, comprises freezing the sample
DNA into a vitreous state, allowing the structure of the
molecules to become visible by electron microscopy. In
contrast, cryo-force spectroscopy employs single-mole-
cules adsorbed on a surface without their solvent mole-
cules. While both techniques use ultra-low temperatures
to reduce thermal motions, only cryo-force spectroscopy
enables imaging and the controlled manipulations of
DNA molecules—cryo-EM is restricted to only structure
“The real asset of cryo-force spectroscopy,” Pawlak
says, “relies on the detection of fine mechanical changes
in a DNA molecule. Our technique could help optimize
DNA nanomachines by properly defining and tuning
the mechanics of the system.”
As mentioned earlier, the term “limit of detection,” is
somewhat of a fallacy in scientific research. When researchers see that, they strive to discover more.
Three researchers were honored with the 2017 Nobel
Prize in Chemistry for their contributions to and advancement of cryo-EM as a scientific technique. The
work they pioneer, described as “decisive for both the
basic understanding of life’s chemistry and for the development of pharmaceuticals” by the Royal Swedish
Academy of Sciences, is never to be understated.
That being said, Pawlak and his team are the first to
successfully use cryo-spectroscopy in research applications where cryo-EM was once considered the standard.
With the proof of concept established, the researchers
have set an eye toward the future, evaluating how they
can use their new method to impact future studies.
Hauer’s colleagues from Italy took a very complex
process, simplified it, and created a compact spectrometer that served a niche application and research group.
The spectrometer, with its ability to employ two lasers
rather than the standard one single, would become the
cornerstone of Hauer’s research.
Ultimately, the specially designed spectrometer allowed the European research team to acquire both
emission and absorption spectra on individual molecules—truly reaching the limits of detectability.
Gold nanoparticles are attached to threads of gold nanowires. Both structures
were formed using a novel redox reaction involving water microdroplets.
Image: Courtesy of Jae Kyoo Lee