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Scientists are constantly pushing the boundaries of the infinitely small… Recently, an international team of chemists reached a new milestone by creating the smallest and densest knot yet, consisting of just 54 atoms. This almost accidental feat could well have a concrete impact on the field of molecular chemistry.
Molecular chemistry, a scientific discipline dedicated to the study and manipulation of molecules, has just reached a new milestone thanks to international researchers from the Chinese Academy of Sciences and the University of Western Ontario.
This advance, described in detail in the journal Nature Communications, is not only a technical masterpiece, but also raises questions about the limits of miniaturization.
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An unexpected discovery
The international research team led by Zhiwen Li, Jingjing Zhang, Gao Li from the Chinese Academy of Sciences and Richard J. Puddephatt from the University of Western Ontario initially worked on a separate project in the field of organic chemistry. The aim was to synthesize metal acetylides, specific chemical compounds used to facilitate various organic reactions. These acetylides are generally involved in the formation of molecular chains or more complex structures in organometallic chemistry.
In their experiments, the researchers tried to link carbon structures with gold acetylides. This process is commonly used to form chains of gold molecules called catenanes. However, something unexpected happened in this experiment: instead of forming a linear chain or the intended structure, the molecules came together to form a trefoil knot. This extremely complex and rare structure in the field of molecular chemistry arose by chance.
Diagram showing the structure of the world's smallest knot. © Li et al., 2024
Consisting of just 54 atoms, this trefoil knot represents a three-dimensional structure in which the atoms are arranged to twist three times in an intertwined loop, with no free ends. The accidental discovery of this structure highlights the unpredictable and often surprising aspect of scientific research, where experiments aimed at a specific goal can sometimes lead to completely unexpected discoveries.
A small knot with a big impact?
The created molecular node is characterized by its exceptionally low Backbone Crossing Ratio (BCR) of 18. BCR is a key indicator in knot chemistry and measures the density and complexity of a molecular knot. The lower the BCR, the tighter and more compact the knot. In the context of organic knots, a typical BCR is between 27 and 33, making the 54-atom knot particularly characterized by its denser and stronger structure than smaller knots previously created.
This unique feature of the knot not only represents a technical advance in the synthesis of complex molecular structures, but also provides valuable insights into natural processes. In fact, similar knotted structures form in vital biological molecules such as DNA and RNA. These knotted structures can influence the stability, activity and function of proteins and thereby play a key role in various biological processes. Certain molecular knots in proteins can make them more resistant to enzymatic degradation, which has important implications for understanding diseases and developing treatments.
Outside the biological context, integrating molecular knots into polymers could lead to plastics with improved mechanical properties such as higher strength or flexibility. These materials could find applications in various fields, from engineering to medicine.
Towards the infinitely small and beyond
The discovery of this small molecular knot approaches what quantum chemical calculations suggest is the theoretical stability limit for this type of structure. According to these calculations, an optimal trefoil structure could be about 50 molecules long. This proximity to the theoretical limit suggests that researchers are reaching the limits of what is possible when manipulating the infinitely small.
The structure of the metal trefoil knot (color code: Au, Red; P, Purple; O, Purple). © Li et al., 2024
The exact method by which this knot was created remains a mystery. This accidental discovery therefore offers insights into the self-organizing processes that can occur at the molecular level, an area that remains largely unexplored.
Continued study of these microscopic structures could not only contribute to the understanding of fundamental mechanisms in biology, such as the folding of proteins or the entanglement of DNA, but also inspire new approaches to the design of advanced materials. Understanding the spontaneous formation of these knots could even lead to innovative methods for producing self-assembling materials and so-called smart materials.
Source: Nature Communications