Through their research efforts, the team was able to finally disprove an
intuitive assumption that in order for two particles of matter to merge and
form larger units (i.e. aggregates or clusters), they must be attracted to
each other. As early as the turn of the century, a team of soft matter
physicists headed by Christos Likos of the University of Vienna predicted on
the basis of theoretical considerations that this does not necessarily have
to be the case. They suggested that purely repulsive particles could also
form clusters, provided they are fully overlapping and that their repulsion
fulfills certain mathematical criteria.
Since then, further theoretical and computational work has demonstrated that
if compressed under external pressure, such clusters develop crystalline
order in a way similar to conventional materials such as copper and
aluminum. Put simply, a crystalline order signifies a periodic lattice
structure in which all particles have fixed positions. In contrast to
metals, however, the particles that form cluster crystals are highly mobile
and continuously jump from one lattice site to the next. This gives these
solids properties that are similar to liquids. Each particle will at some
point be found at each lattice site.
Particles with pompom-like structure
It proved difficult to produce particles that had the necessary
characteristics for the detection of cluster crystals. However, Emmanuel
Stiakakis from Forschungszentrum Jülich and his colleagues have now
succeeded in achieving this aim in close collaboration with theoreticians
from Vienna and polymer chemists from Siegen. The researchers were able to
produce hybrid particles with a pompom-like structure. The core of these
particles is comprised of organic polymers to which DNA molecules are
attached and which stick out in all directions like the threads of a pompom.
This structure enables the molecules to be pushed far inside each other and
thus to be sufficiently compressed. At the same time, the combination of an
electrostatic repulsion of naturally charged DNA components and a weak
interaction of polymers at the center of the constructs ensures the
necessary overall interaction.
"DNA is particularly well suited for our intentions, as it can be assembled
relatively easily in the desired shape and size due to the Watson–Crick base
pairing mechanism. In combination with polymer cores, the shape and
repulsion of the hybrid particles can be fine-tuned and different variations
can be produced relatively quickly," explains Stiakakis, who conducts
research at Forschungszentrum Jülich's Institute of Biological Information
Processing. The physicist with a Ph.D. in the field of physical chemistry
has long been using these helix molecules to investigate aspects of
self-assembling soft matter.
"After extensive efforts and by applying numerous experimental methods,
including biochemical synthesis and characterization as well as X-ray
scattering and light scattering, we have now been able to bring a more than
20-year search for cluster crystals to a successful conclusion," says a
delighted Likos. The theoretical physicist at the University of Vienna's
Faculty of Physics now anticipates the discovery of further complex states
of matter, which will be formed by the new macromolecular aggregates.
Reference:
Emmanuel Stiakakis et al, Self assembling cluster crystals from DNA based
dendritic nanostructures, Nature Communications (2021).
DOI: 10.1038/s41467-021-27412-3
Tags:
Physics