Scientists now report the synthesis of the most complex non-DNA molecular knot prepared to date. They created a 160-atom-loop with five crossing points, a molecular pentafoil knot. The molecular knot created is a self-assembly of five bis-aldehyde and five bis-amine building blocks about five metal cations (Iron) and one chloride anion.

The research results were reported this week in the journal Nature Chemistry.



The structure of the molecular pentafoil knot made by the researchers. Image Credit: Robert W. McGregor, mcgregorfineart.com

Knots can be found in DNA, proteins and even in the molecules that make up man-made plastics, where they often play an important role in the substance's properties (for example, 85% of the elasticity of natural rubber is due to knot-like entanglements in the rubber molecules chains).

General structure of a pentafoil knot

However, deliberately tying molecules into well-defined knots so that these effects can be studied is extremely difficult. Up to now, only the simplest type of knot – a trefoil knot - had been prepared by scientists. Now Professor David Leigh's team at the University of Edinburgh together with Academy Professor Kari Rissanen at the University of Jyväskylä have succeeded in preparing and characterizing a more complex type of knot – a pentafoil knot (also known as a cinquefoil knot or a Solomon's seal knot) - a knot which looks like a five-pointed star.

Remarkably, the thread that is tied into the star-shaped knot is just 160 atoms in length – that is about 16 nanometers long (one nanometer is one millionth of a millimeter). The Edinburgh researchers used a technique known as "self-assembly" to prepare the knot in a chemical reaction. The building blocks are chemically programmed to spontaneously wrap themselves up into the desired knot. Making knotted structures from simple chemical building blocks in this way should make it easier to understand why entanglements and knots have such important effects on material properties and may also help scientists to make new materials with improved properties based on knotted molecular architectures.



A DNA molecule pentafoil knot (left - viewed through an electron microscope; from a 80,000x magnification image). Image Credit: Robert W. McGregor, mcgregorfineart.com.

David Leigh, Edinburgh University professor of organic chemistry, said: "It's very early to say for sure, but the type of mechanical cross-linking we have just carried out could lead to very light but strong materials, something akin to a molecular chain mail."

"It could also produce materials with exceptional elastic or shock-absorbing properties because molecular knots and entanglements are intimately associated with those characteristics.

"By understanding better how those structures work and being able to create them to order we should be able to design materials that exploit those architectures with greater effect."